Showing posts with label New Learning Composite mathematics. Show all posts
Showing posts with label New Learning Composite mathematics. Show all posts

S.chand publication New Learning Composite mathematics solution of class 8 Chapter 12 Mensuration Exercise 12C

 Exercise 12C


Q1 | Ex-12C |Class 8 |Mensuration | S.Chand | New Learning | Composite mathematics | myhelper

Question 1

Find the surface area and lateral area of each cuboid / cube. All lengths are in centimetres.

(a)







Surface Area=6×(side)2

=6×(10)2

=6×100=600unit2


Lateral Area=4×(side)2

=4×(10)2

=4×100

=400 unit2


(b)








S.A=2(l×b + b×h + h×l)

=2(10×6 + 6×4 + 4×10)

=2(60+24+40)

=2×124

=248cm2


L.A=2h(l+b)

=2×4×(10+6)

=8×(16)
=128cm2


(c)







S.A=6×(side)2

=6×(3)2 

=6×9

=54cm2


L.A=4×(side)2

=4×(3)2

=4×9

=36cm2


(d)











S.A=2(l×b + b×h + h×l)

=2(15×15 + 15×12 + 12×15)

=2(225+180+180)

=2×585

=1170 cm2


L.A=2h(l+b)

=2×12(15+15)

=2×12×30

=720 cm2


(e)






Sol :

S.A=2(l×b + b×h + h×l)

=2(17×11+11×12 +12×17)

=2×(187+132+204)
=2×(523)
=1046 cm2

L.A=2h(l+b)

=2×12×(17+11)

=24×28

=672 cm2


(f)

S.A=6×(side)2

=6×(4.5)2

=6×20.25

=121.50 cm2


L.A=4×(side)2

=4×(4.5)2

=4×20.25

=81 cm2



Q2 | Ex-12C |Class 8 |Mensuration | S.Chand | New Learning | Composite mathematics | myhelper

Question 2

Find the surface area of a cuboid, given

(a) Length=8 cm, Breadth=7 cm, Height=12 cm

(b) Length=60 mm, Breadth=40 mm, Height=55 mm

Sol :

(a)







L=8cm , B=7cm, H=12cm

∴S.A={2(8×7)}+{2(7×12)}+{2(8×12)}

=(2×56)+(2×84)+(2×96)

=112+168+192=472cm2


(b)







L=60cm , B=40cm , H=55cm

∴S.A={2(60×55)}+{2(40×55)}+{2(60×40)}
=(2×3300)+(2×2500)+(2×2400)
=6600+4800+4800=15800cm2



Q3 | Ex-12C |Class 8 |Mensuration | S.Chand | New Learning | Composite mathematics | myhelper

Question 3

Find the surface area of a cube with side length

(a) 6 cm







S.A={2(6×6)}+{2(6×6)}+{2(6×6)}

=(2×36)+(2×36)+(2×36)

=72+72+72=216cm2


(b)






=(2×324)+(2×324)+(2×324)

=648+648+648=1944cm2


(c)







S.A={2(1.7×1.7)}+{2(1.7×1.7)}+{2(1.7×1.7)}

=(2×2.89)+(2×2.89)+(2×2.89)

=5.78+5.78+5.78=17.34cm2



Q4 | Ex-12C |Class 8 |Mensuration | S.Chand | New Learning | Composite mathematics | myhelper

Question 4

Find the surface area of a wooden cube of side length 30cm

Sol :






Cube side=30cm

S.A={2(30×30)}+{2(30×30)}+{2(30×30)}

=(2×900)+(2×900)+(2×900)

=1800+1800+1800=5400cm2



Q5 | Ex-12C |Class 8 |Mensuration | S.Chand | New Learning | Composite mathematics | myhelper

Question 5

A cornflakes box measures 30 cm by 24 cm by 8 cm. Calculate the surface area of the box

Sol :


L=24cm, B=8cm ,H=30cm

S.A={2(24×8)}+{2(24×30)}+{2(30×8)}

={2×192}+{2×720}+{2×240}

=384+1440+480=2304cm2



Q6 | Ex-12C |Class 8 |Mensuration | S.Chand | New Learning | Composite mathematics | myhelper

Question 6

Find the missing dimension in the figure with the given surface area.

Sol :




B=5cm ,H=12cm ,L=?

S.A={2(x×5)}+{2(x×12)}+{2(5×12)}

={5x×2}+{24x}+{2×60}

=10x+24x+120=3x+120

ATQ,

S.A=256=34x+120

34x=256-120=136

$x=\frac{136}{34}$=4cm



Q7 | Ex-12C |Class 8 |Mensuration | S.Chand | New Learning | Composite mathematics | myhelper

Question 7

A cuboid is 3 cm by 4 cm by 7 cm. Explain whether doubling all of the dimensions would double the surface area?

Sol :




L=4cm, B=3cm ,H=7cm

S.A1={2(4×3)}+{2(4×7)}+{2(3×7)}

={2×12}+{2×28}+{2×21}

=24+56+42=122cm2


If doubling all of dimensions 

∴L=4×2=8cm ,B=3×2=6cm ,H=7×2=14cm

∴S.A2={2(8×6)}+{2(6×14)}+{2(8×14)}

=(2×48)+(2×84)+(2×112)

=96+168+224=488


∴S.A2=488

S.A1=122

∴S.A2=122×4=S.A1×4

∴No, It will be 4 times



Q8 | Ex-12C |Class 8 |Mensuration | S.Chand | New Learning | Composite mathematics | myhelper

Question 8

The net consists of 6 equal squares with side length 8 cm







(a) Name the solid it represents.

(b) Find the surface ares of the solid.

Sol :









(a) The solid shape is cube

(b) S.A={2(8×8)}+{2(8×8)}+{2(8×8)}

={2×64}+{2×64}+{2×64}=128+128+128=384cm2



Q9 | Ex-12C |Class 8 |Mensuration | S.Chand | New Learning | Composite mathematics | myhelper

Question 9

A small indoor greenhouse (herbarium) is made entirely of glass panes (including base) held together with tape. It is 30 cm long. 25 cm wide and 25 cm high.







(a) What is the area of glass?

(b) How much tape is needed for all the 12 edges?

Sol :

(a)

S.A.={2(30×25)}+{2(30×25)}+{2(25×25)}

=(2×750)+(2×750)+(2×625)

=1500+1500+1250=4250cm2


(b)

Length of top for all 12 edges=Perimeter of the top+Four vertical edge

={2(30+25)}+{25×4}

=(2×55)+(100)

=110+100=210cm



Q10 | Ex-12C |Class 8 |Mensuration | S.Chand | New Learning | Composite mathematics | myhelper

Question 10

Two cubes each of 10 cm edge are joined end to end. Find the surface area of the resulting cuboid.

Sol :



S.A={2(20×10)}+{2(20×10)}+{2(10×10)}

=(2×200)+(2×200)+(2×100)

=400+400+200=1000cm2



Q11 | Ex-12C |Class 8 |Mensuration | S.Chand | New Learning | Composite mathematics | myhelper

Question 11

Daniel is painting the walls and ceiling of a cuboidal hall with length, breadth and height of is 15 m, 10 m and 7 m respectively. From each can of paint, $100 \mathrm{~m}^{2}$ of area is painted. Hone many cans of paint will he need to paint the entire room?

Sol :









S.A={2(15×7)}+{2(10×7)}+{2(15×10)}

=(107×2)+(70×2)+(150)

=210+140+150=500cm2

∴Cans of paint needed$=\frac{500}{100}=5$



Q12 | Ex-12C |Class 8 |Mensuration | S.Chand | New Learning | Composite mathematics | myhelper

Question 12

A cuboidal tin box opened at the top has dimensions 20cm×16cm×14cm. What is the total area of metal duct sheet required to make 10 such boxes?

Sol :







S.A=2(l×b + b×h + h×l)

=2(20×16+16×14+14×20)

=2×(320+224+280)

=2×824

=1648 cm2

∴1 box need 1648 cm2 sheet

∴10 box need 1648×10=16480cm2 sheet



Q13 | Ex-12C |Class 8 |Mensuration | S.Chand | New Learning | Composite mathematics | myhelper

Question 13

Find the area to be painted in the given block with a cylindrical hole. Given that length is 15 cm, width is 12 cm, height is 20 cm and radius of the holes is 2.8 cm.

Sol :




S.A=2(l×b + b×h + h×l)

=2(15×12 + 12×20 + 20×15)

=2(180+240+300)

=2×720

=1440 cm2

∴Area of hole=πr2

$=\frac{22}{7}\times (2.8)^2$

$=\frac{22}{7}\times \frac{28}{10}\times \frac{28}{10}$

$=\frac{2464}{100}$

=24.64

∴Area to be painted=Total surface of the block-(2×Area of one circular hole)

=1440-(24.64×2)

=1440-49.28

=1390.72cm2



Q14 | Ex-12C |Class 8 |Mensuration | S.Chand | New Learning | Composite mathematics | myhelper

Question 14

The area of the four walls in a room is $120 \mathrm{~m}^{2}$. The length is twice the breadth. If the height of the room is 4m , find the area of the floor.

Sol :
Let breadth of room =x m
Length of room =2x m
Height of the rm=4 m
The area of the four walls=2(bh+lh)=120
2(x×4+2x×4)=120
2(12x)=120
$x=\frac{120}{4}$=5 m

Breadth=x=5 m , 
Length=2x=2×5=10 m
Area of floor=l×b=10×5=50 m2



Q15 | Ex-12C |Class 8 |Mensuration | S.Chand | New Learning | Composite mathematics | myhelper

Question 15

A cube of edge 5 cm is cut into cubes, each of edge 1 cm. Find the ratio of the total surface area of one of small cubes to that of the large cube.
Sol :
Edge of large cube =5 cm
Area of large cube $=6(a)^{2}=6 \times(5)^{2}$
=6×25=150 cm2

Edge of small cube=1 cm

Area of small cube$=6 \times(a)^{2}=6 \times(1)^{2}$

$=6 \times 1=6$


Area of small cube : Area of large cube $=\frac{6}{150}=\frac{1}{25}$
1 : 25


S.chand publication New Learning Composite mathematics solution of class 8 Chapter 6 Factorisation of Algebraic Expressions Exercise 6C

 Exercise 6C


Q1 | Ex-6C | Class 8 | S.Chand | New Learning Composite maths | chapter 6 |Factorisation of Algebraic Expressions | myhelper

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Question 1

ax2 + by2 + bx2 + ay2

Sol :

=ax2+by2+bx2+ay2

=x2(a+b)+y2(a+b)

=(a+b)(x2+y2)



Q2 | Ex-6C | Class 8 | S.Chand | New Learning Composite maths | chapter 6 |Factorisation of Algebraic Expressions | myhelper

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Question 2

my + nz + mz + ny

Sol :

=my+mz+nz+ny

=m(y+z)+n(y+z)

=(y+z)(m+n)



Q3 | Ex-6C | Class 8 | S.Chand | New Learning Composite maths | chapter 6 |Factorisation of Algebraic Expressions | myhelper

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Question 3

ab – 5c + 5a – bc

Sol :

=ab+5a+5c-bc

=ab-bc-5c+5a

=b(a-c)+5(a-c)

=(a-c)(b+5)



Q4 | Ex-6C | Class 8 | S.Chand | New Learning Composite maths | chapter 6 |Factorisation of Algebraic Expressions | myhelper

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Question 4

x2 + ab – ax – bx

Sol :

=x2+ab–ax–bx

=x(x-a)-b(x-a)
=(x-a)(x-b)



Q5 | Ex-6C | Class 8 | S.Chand | New Learning Composite maths | chapter 6 |Factorisation of Algebraic Expressions | myhelper

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Question 5

ab2 – bc2 – ab + c2

Sol :

=ab(b-1)-c2(b-c)

=(b-1)(ab-c2)



Q6 | Ex-6C | Class 8 | S.Chand | New Learning Composite maths | chapter 6 |Factorisation of Algebraic Expressions | myhelper

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Question 6

2xw – 3 – 6x + w

Sol :

=2xw+w-3-6x

=w(2x+1)-3(2x+1)

=(2x+1)(w-3)



Q7 | Ex-6C | Class 8 | S.Chand | New Learning Composite maths | chapter 6 |Factorisation of Algebraic Expressions | myhelper

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Question 7

54x + 10y – 12xy – 45

Sol :

=54x-45+10y-12xy

=9(6x-5)-2y(6x-5)

=(9-2y)(6x-5)



Q8 | Ex-6C | Class 8 | S.Chand | New Learning Composite maths | chapter 6 |Factorisation of Algebraic Expressions | myhelper

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Question 8

m3 + 4m + 5m2 + 20

Sol :

=m3+5m2+4m+20

=m2(m+5)+4(m+5)

=(m2+4)(m+5)



Q9 | Ex-6C | Class 8 | S.Chand | New Learning Composite maths | chapter 6 |Factorisation of Algebraic Expressions | myhelper

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Question 9

2axy2 + 10x + 3ay2 + 15

Sol :

=2axy2+10x+3ay2+15

=ay2(2x+3)+5(2x+3)

=(ay2+5)(2x+3)

S.chand publication New Learning Composite mathematics solution of class 8 Chapter 2 Exponents Exercise 2A

 Exercise 2A


Q1 | Ex-2A |Class 8 |Exponents | S.Chand | New Learning | Composite maths | myhelper

Question 1

Evaluate

(a) 90

Sol : 1


(b) $\left(\frac{1}{3}\right)^0$

Sol : 1


(c) $\left(-\frac{64}{21}\right)^0$

Sol : 1


(d) (395.008)0

Sol : 1


(e) $\left(\frac{3xy}{ab}-z\right)^0$

Sol : 1


(f) $\frac{1}{a^0 + b^0 + c^0}$

Sol : $=\frac{1}{3}$

(g) (80 – 20)x 250

Sol : 

=(1-1)×1

=0×1=0


(h) (350 + 20 + 30) ÷ 3

Sol : 1



Q2 | Ex-2A |Class 8 |Exponents | S.Chand | New Learning | Composite maths | myhelper

Question 2

Simplifying giving the answer as a fraction:
(a) 6-1

Sol : $=\frac{1}{6}$

(b) 10-1

Sol : $=\frac{1}{10}$

(c) 4-3

Sol : $=\frac{1}{4^3}=\frac{1}{64}$


(d) 2-7

Sol : 
$=\frac{1}{2^7}=\frac{1}{128}$


(e) 3-3

Sol : 
$=\frac{1}{3^3}=\frac{1}{27}$


(f) p-1

Sol : 
$=\frac{1}{p}$


(g) (x-2)2

Sol : 
$=\frac{1}{x^4}$


(h) (10a)-2

Sol : 
$=\frac{1}{100a^2}$


(i) (x+y)-2

Sol : 
$=\frac{1}{(x+y)^2}$


(j) (m5)-3

Sol : 
$=\frac{1}{m^{15}}$



Q3 | Ex-2A |Class 8 |Exponents | S.Chand | New Learning | Composite maths | myhelper

Question 3

Find the reciprocal of the following.

(a) (3)-2

Sol : 
$=\frac{1}{3^2}=\frac{1}{9}$


(b) (-2)-5

Sol : 
$=\frac{1}{(-2)^5}=-\frac{1}{32}$


(c) $\left(\frac{2}{5}\right)^{-3}$
Sol : 

$=\left(\frac{5}{2}\right)^3=\frac{125}{8}$


(d) $\left(\frac{2}{3}\right)^{-4}$

Sol : 

$=\left(\frac{3}{2}\right)^4=\frac{81}{16}$



Q4 | Ex-2A |Class 8 |Exponents | S.Chand | New Learning | Composite maths | myhelper

Question 4

Simplify and express the result in power notation with positive exponent.
(a) (-2)-4 x (-2)-6

Sol : $=\frac{1}{-2^4}\times \frac{1}{-2^6}=\frac{1}{2^{10}}$

(b) 37 x 3-9 x 36

Sol : $=\frac{1}{3^7}\times \frac{1}{3^9}\times \frac{1}{3^{6}}$
=34

(c) $3^7\times \left(\frac{3}{5}\right)^{-7}$

Sol : $=3^7 \times \left(\frac{5}{3}\right)^7=3^7 \times  \frac{5^7}{3^{7}}$

=57

(d) 2-7 x [(2)-3]-4

Sol : $=\frac{1}{2^7}\times 2^{12}$

=25

(f) (4-8 ÷ 4-10) x 4-5

Sol : $=\frac{4^{-8}}{4^{-10}}\times 4^{-5}$ $=\frac{1}{4^2}\times 4^{-5}=4^{-3}=\frac{1}{4^3}$



Q5 | Ex-2A |Class 8 |Exponents | S.Chand | New Learning | Composite maths | myhelper

Question 5

Evaluate:
(a) (80 + 5-1)×$\left(\frac{1}{5}\right)^{-2}$

Sol :

$=\left(1+\frac{1}{5}\right)\times (5^{-1})^{-2}$

$=\frac{5+1}{5}\times 5^2$

$=\frac{6}{5}\times 5^2$

=6×5=30


(b) (300+ 150)×$\left(\frac{1}{2}\right)^{-3}$

Sol :

=(1+1)×(2-1)-3=2×23

=24=16


(c) (7 -1+ 8-1 + 9-1)0÷$\left(-\frac{1}{9}\right)^{-2}$

Sol :

$=\left(\frac{1}{7}+\frac{1}{8}+\frac{1}{9}\right)^0 \div \left(\frac{9}{-1}\right)^2$
$=1\div \frac{81}{1}=\frac{1}{81}$

(d) $\left[\left(\frac{1}{3}\right)^{-2}+\left(\frac{1}{4}\right)^{-2}+\left(\frac{1}{6}\right)^{-2}\right]$

Sol :

=32+42+62

=9+16+36

=25+36=61


(e) $\left\{\left(-\frac{4}{5}\right)^{-2}\right\}^{-2}$

Sol :

$=\left\{\left(-\frac{5}{4}\right)\right\}^{-2}=\left(-\frac{5}{4}\right)^{-4}$

$=\left(\frac{4}{5}\right)^4=\frac{256}{625}$


(f) $9^{-1}\times \frac{4^3}{3^{-4}}$

Sol :

$=\frac{(3^2)^{-1}\times 4^3}{3^{-4}}=\frac{3^{-2}\times 4^3}{3^{-4}}$

$=\frac{4^3}{3^{-2}}$

=43×32=576


(g) $(5^{-1}\times 4^{-1})\times \left(\frac{3}{2}\right)^{-2}$

Sol :

$=\left(\frac{1}{5}\times \frac{1}{4}\right)\times \left(\frac{2}{3}\right)^2$

$=\frac{1}{20}\times \frac{2^2}{3^2}$

$=\frac{1}{20}\times \frac{4}{9}=\frac{1}{45}$



Q6 | Ex-2A |Class 8 |Exponents | S.Chand | New Learning | Composite maths | myhelper

Question 6

Evaluate:

(a) $\left[\left(\frac{1}{4}\right)^{-1}-\left(\frac{1}{5}\right)^{-1}\right]+1$
Sol :

=(4-5)+1

=-1+1=0


(b) $\left[\left(\frac{11}{12}\right)^{-10} \div \left(\frac{11}{12}\right)^{-12}\right]\times 1\frac{23}{121}$

Sol :

$=\left[\left(\frac{12}{11}\right)^{10}\div \left(\frac{12}{11}\right)^{12}\right]\times \frac{144}{121}$

$=\left(\frac{12}{11}\right)^{-2}\times \left(\frac{12}{11}\right)^{2}$

$=\frac{(11)^2}{(12)^2}\times \frac{(12)^2}{(11)^2}$

=1



Q7 | Ex-2A |Class 8 |Exponents | S.Chand | New Learning | Composite maths | myhelper

Question 7

Find the value of m for which, 72m ÷ 7-4 = 7-6

Sol :

$=\frac{7^{2m}}{7^{-4}}=$7-6

⇒72m=7-6×7-4=7-10

⇒(72)m=(72)-5

⇒m=-5

S.chand publication New Learning Composite mathematics solution of class 8 Chapter 1 Rational numbers Exercise 1F

 Exercise 1F


Q1 | Ex-1F |Class 8 |Rational Numbers | S.Chand | New Learning | Composite maths | myhelper

Question 1

A group of friends hike $5\frac{3}{4}$ km, stops for lunch, and then hike another $3\frac{1}{5}$ km. How far did they hike?

Sol :

Before lunch hiking distance $=5\frac{3}{4}$km $=\frac{23}{4}$ km

After lunch hiking distance $=3\frac{1}{5}=\frac{16}{5}$ km 

Total distance$=\frac{23}{4}+\frac{16}{5}=\frac{115+64}{20}$

$=8\frac{19}{20}$km



Q2 | Ex-1F |Class 8 |Rational Numbers | S.Chand | New Learning | Composite maths | myhelper

Question 2

Ms Joshi walks her dog $\frac{4}{5}$ km each day. What is the total distance that Ms Joshi walks her dog in 6 days?

Sol :

Ms Joshi walks her dog $\frac{4}{5}$ km each day

Total distance that Ms Joshi walks her dog in 6 days$=6\times \frac{4}{5}=\frac{24}{5}$ $=4\frac{4}{5}$km



Q3 | Ex-1F |Class 8 |Rational Numbers | S.Chand | New Learning | Composite maths | myhelper

Question 3

Priya completes $\frac{1}{20}$ of her painting each day. How much of her painting does she complete 5 days?

Sol :

Priya completes $\frac{1}{20}$ of her painting each day

Priya completes a portion of painting in 5 day $=5\times \frac{1}{20}=\frac{1}{4}$



Q4 | Ex-1F |Class 8 |Rational Numbers | S.Chand | New Learning | Composite maths | myhelper

Question 4

An oxygen tank contained $219\frac{2}{3}$ L of oxygen before $32\frac{1}{3}$ L were used. If the tank can hold $245\frac{3}{8}$ L, how much space in the tank is unused?

Sol :

An oxygen tank contained $219\frac{2}{3}=\frac{659}{3}$ L 

Oxygen used$=32\frac{1}{3}=\frac{97}{3}$ L

Capacity of tank$245\frac{3}{8}=\frac{1963}{8}$ L

Space unused in the tank $=\frac{1963}{8}-\left[\frac{659}{3}-\frac{97}{3}\right]$

$=\frac{1963}{8}-\frac{562}{3}=\frac{5883-4432}{24}$

$=\frac{1393}{24}=58\frac{1}{24}$ L



Q5 | Ex-1F |Class 8 |Rational Numbers | S.Chand | New Learning | Composite maths | myhelper

Question 5

A water pipe has an outside diameter of 3.5 cm and a wall thickness of $\frac{25}{32}$ cm. What is the inside diameter of the pipe?







Sol :

Let ,

Inner diameter(d)=x

Outer diameter(D)=3.5cm

Thickness(t)$=\frac{257}{32}$cm

$t=\frac{3.5-d}{2}$

or $\frac{25}{32}=\frac{3.5-d}{2}$

or 3.5-d$=\frac{25\times 2}{32}$

or -d$=\frac{25}{16}-\frac{35}{10}$

or $d=\frac{35}{10}-\frac{25}{16}=\frac{560-250}{160}$

$=\frac{310}{160}=1\frac{15}{16}$cm



Q6 | Ex-1F |Class 8 |Rational Numbers | S.Chand | New Learning | Composite maths | myhelper

Question 6

 Find the perimeter and area of a rectangular piece of land measuring $12\frac{1}{2}$m by $5\frac{1}{5}$ m. How much area of the land is left unused if 4 small square flower beds of side length 1 m are made at the center?

Sol :






Length of rectangle(L)$=12\frac{1}{2}$cm  $=\frac{25}{2}$cm

Breadth of rectangle (B)$=5\frac{1}{5}$cm $=\frac{26}{5}$ cm

Perimeter of rectangle=2(L+B)

$=2\left(\frac{25}{2}+\frac{26}{5}\right)$

$=2\left(\frac{125+52}{10}\right)=2\times \frac{177}{10}$

$=\frac{177}{5}=35\frac{2}{5}$cm

∴Area of rectangle(L×B)$=\left(\frac{25}{2}\times \frac{26}{5}\right)$

=65cm2

∴The flowered area=12=1cm2

∴4 flowered bed area=4×1=4cm2

∴Area of the land is left unused=(65-4)=61cm2



Q7 | Ex-1F |Class 8 |Rational Numbers | S.Chand | New Learning | Composite maths | myhelper

Question 7

Out of a certain sum of money a boy spends $\frac{3}{5}$, and then $\frac{1}{4}$ of the remainder. He has Rs 15 left. How much amount had he at first?

Sol :

Total money=x

Money spend $=\frac{3x}{5}$

Remainder$=\frac{1}{4}\left(x-\frac{3x}{5}\right)$

ATQ,

$x-\left[\frac{3x}{5}+\frac{1}{4}\left(x-\frac{3x}{5}\right)\right]=15$

or $x-\left[\frac{3x}{x}+\frac{1}{4}\left(\frac{5x-3x}{5}\right)\right]=15$

or $x-\left[\frac{3x}{5}+\frac{2x}{20}\right]=15$

or $x-\left[\frac{12x+2x}{20}\right]=15$

or $x-\frac{14x}{20}=15$

or $\frac{20x-14x}{20}=15$

or 6x=15×20

or $x=\frac{515\times 20}{6}=50$

Ans 50



Q8 | Ex-1F |Class 8 |Rational Numbers | S.Chand | New Learning | Composite maths | myhelper

Question 8

$10\frac{1}{2}$ tonnes of sand are to be shared between a number of builders. One of them receives $\frac{4}{7}$ of the total and the remaining sand is shared by 3 builders. How much does the fourth builder receive and how much sand does each of the other 3 builders receive?

Sol :

Total sand $=10\frac{1}{2}=\frac{21}{2}$ tonnes

Fourth builder get$=\frac{21}{2}\times \frac{4}{7}$=6 tonnes

Rest sand$=\frac{21}{2}-6=\frac{21-12}{2}=\frac{9}{2}$

Distributing into 3$=\frac{9}{2\times 3}=\frac{3}{2}$ tonnes 



Q9 | Ex-1F |Class 8 |Rational Numbers | S.Chand | New Learning | Composite maths | myhelper

Question 9

A man aged 27 marries a woman aged 24, he dies at the age of 81 and she dies at the age of 91. For what fraction of his life is the man married? For what fraction of her life is the woman a widow?

Sol :

Man married life duration$=\frac{81-27}{81}=\frac{2}{3}$

Woman widow life duration$=\frac{91-(54+24)}{91}=\frac{91-48}{91}$ $=\frac{13}{91}=\frac{1}{7}$



Q10 | Ex-1F |Class 8 |Rational Numbers | S.Chand | New Learning | Composite maths | myhelper

Question 10

Shalini has to cut out circles a diameter $1\frac{1}{4}$ cm from an aluminium strip of dimensions $8\frac{3}{4}$ cm by $1\frac{1}{4}$ cm. How many full circles can Shalini cut? Also calculate the wastage of the aluminium strip.

Sol :

Circle diameter(d)$=1\frac{1}{4}$cm

Strip length$=8\frac{3}{4}=\frac{35}{4}$cm

Strip breadth$=1\frac{1}{4}=\frac{5}{4}$

∴Number of full circles which can be cut from the strip$=\frac{35}{4}\div \frac{5}{4}$

$=\frac{35}{4}\times \frac{4}{5}$

=7


$r=\dfrac{\frac{5}{4}}{2}=\frac{5}{8}$


Area of circle=πr2=$\pi\times \frac{5}{8}\times \frac{5}{8}$

$=\frac{22}{7} \times \frac{5}{8}\times \frac{5}{8}$cm2


Area of 7 circles $=7\times\frac{22}{7} \times \frac{5}{8}\times \frac{5}{8}$

$=\frac{275}{32}$cm2


Area of strip$=\frac{35}{4}\times \frac{5}{4}=\frac{175}{16}$cm2

∴Wastage=(Area of strip-Area of 7circles)

$=\frac{175}{16}-\frac{275}{32}$

$=\frac{175\times 2-275}{32}=\frac{350-275}{32}$

$=\frac{75}{32}=2\frac{11}{32}$ cm2

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