EXERCISE 11
Q1 | Ex-11 | Cl-8 | Schand Composite Mathematics | Time and Work | Myhelper
Question 1
A can do 34 of the work in 12 days . In how many days can he complete 18th of the work ?
Sol :
⇒A can do 34 of work in 12 days
⇒ A can do 1 work in 12×43 days And [Multiplying both side by 4/3]
⇒ A can do 18 work in 12×43×18 days [Multiplying both side by 1/8]
=4×12 days
= 2 days
Q2 | Ex-11 | Cl-8 | Schand Composite Mathematics | Time and Work | Myhelper
Question 2
Tanu can do a piece of work in 24 days and Manisha can do it in 30 days . How long will they take to do the work working together ?
Sol :
⇒Tanu can do a piece of work in 24 days
⇒So Tanu's one day work =124
⇒Manisha can do the same work in 30 days
⇒So Manisha's one day work =130
⇒We know that
⇒Tanu and Manisha's one day work = Tanu's one day work + Manisha's one day work
=124+130 [Taking LCM]
=5+4120
=9120
=340
⇒Tanu and Manisha's one day work =340
⇒Total time required for completing work 1one day work=1340
⇒So Tanu and Manisha can complete the same work in =403=1313 days.
Q3 | Ex-11 | Cl-8 | Schand Composite Mathematics | Time and Work | Myhelper
Question 3
A and B can polish the floors of a building in 18 days . A alone can do 16 of this job in 4 days . in how many days can B alone polish the floors ?
Sol :
⇒A alone complete whole work in =416=4×61 = 24 days
⇒A's one day work =124
⇒(A and B)'s one day work =118
⇒Also , (A+B)'s one day work = A's one day work + B's one day work
⇒118=124+1x
⇒4−372=1x
⇒172=1x
⇒x = 72
72 days
Q4 | Ex-11 | Cl-8 | Schand Composite Mathematics | Time and Work | Myhelper
Question 4
A and B can weave a certain number of baskets in 12 days and 15 days respectively . They work together for 5 days and then B leaves . In how many days will A finish the rest of the work ?
Sol :
⇒A's one day work =112
⇒B's one day work =115
⇒(A + B)'s one day work = A's one day work + B's one day work
=112+115
=5+460
=960
=320
⇒(A + B)'s one day work =320
⇒They work together for 5 days =5×320=34
⇒Remaining work =1−34
=4−34=14
⇒A can complete all work in 12 days . So , remaining work can be done in =14×12 days
= 3 days
Q5 | Ex-11 | Cl-8 | Schand Composite Mathematics | Time and Work | Myhelper
Question 5
A , B and C working together take 8 min . to address a pile of envelopes . A and B together would take 10 min ; A and C together would take 15 min . How long would each take working alone ?
Sol :
⇒(A + B + C)'s one unit work =18
⇒(A + B)'s one unit work =110
⇒(A + C)'s one unit work =115
⇒C's one unit work = (A + B + C)'s one unit work - (A + B)'s one unit work
=18−110
=5−440=140
∴C alone take 40 min
⇒B's one unit work = (A + B + C)'s one unit work - (A + C)'s one unit work
=18−115
=15−8120=7120
∴B alone take 1207 min
⇒A's one unit work = (A + B + C)'s one unit work - B's one unit work - C's one unit work
=18−7120−140
=15−7−3120=5120=124
∴A alone take 24 min
A→24 min ; B→1207 min ; C→40 min
Q6 | Ex-11 | Cl-8 | Schand Composite Mathematics | Time and Work | Myhelper
Question 6
A and B can do a piece of work in 6 days and 4 days respectively . A started the work ; worked at it for 2 days and then was joined by B . Find the total time taken to complete the work .
Sol :
⇒A's one day work =16
⇒B's one day work =14
⇒A's 2 days work =2×16=13
⇒Remaining work =1−13
=3−13=23..(i)
⇒(A+B)'s one day work = A's one day work + B's one day work
=16+14
=2+312=512
or
⇒Total time required to complete work together=1one day work =1512=125
⇒Time to complete the remaining work together =23×125 =85..(ii)
⇒Total time required to complete work = A's alone working time + (A+B)'s working together time
=2+85
=10+85
=185=335
Q7 | Ex-11 | Cl-8 | Schand Composite Mathematics | Time and Work | Myhelper
Question 7
780 is paid for a task which A can do in 6 days , B in 8 days and C in 4 days . If all work together , how much money should each receive ?
Sol :
⇒A's one day work =16
⇒B's one day work =18
⇒C's one day work =14
∴Ratio of shares of money of A , B and C = Ratio of their one day work
=16:18:14 =16×24:18×24:14×24 [L.C.M of 6 , 8 and 24]
= 4 : 3 : 6
∴Sum of the ratios = 4 + 3 + 6 = 13
⇒A's share =413×780 = 240
⇒B's share =313×780 = 180
⇒C's share =613×780 = 360
A→240 , B→180 , C→360
ALTERNATE METHOD
⇒A's one day work =16
⇒B's one day work =18
⇒C's one day work =14
Total work done=16+18+14
=4+3+624=1324
∴Shares of money of A , B and C = Ratio of their one day work / Total work × Total money
⇒A's share 161324×780
=413×780 = 240
⇒B's share 181324×780
=313×780 = 180
⇒C's share 141324×780
=613×780 = 360
A→240 , B→180 , C→360
Q8 | Ex-11 | Cl-8 | Schand Composite Mathematics | Time and Work | Myhelper
Question 8
X works thrice as fast as Y . If Y alone can complete a job in 18 days , find how ,many days will X and Y together take to complete the job .
Sol :
⇒Y's one day work =118
A.T.Q
⇒3X=Y
⇒X=Y3
⇒X=183
⇒X=6
⇒X's one day work =16
⇒(X+Y)'s one day work = X's one day work + Y's one day work
=16+118
=3+118=418=29
⇒Total time required by (X+Y) to complete work =1one day work =129 =92 or =412 days
Q9 | Ex-11 | Cl-8 | Schand Composite Mathematics | Time and Work | Myhelper
Question 9
A can do a job in 20 days , B in 30 days and C in 60 days . If A is helped by B and C on every third day , then in how many days will the job be finished ?
Sol :
⇒A's one day work =120..(i)
⇒(A+B+C)'s one day work = A's one day work + B's one day work + C's one day work
=120+130+160
=3+2+160
=660=110..(i)
⇒According to question, A work starting two days alone and on every third day all together work .
Total time to complete work = 1st day + 2nd day + 3rd day
1 and 2 day equal to (i) and 3 day equal to (ii)
=120+120+110
=1+1+220=420=15
Which means in 3 days 1/5 work is completed
3 days=15 work
3×5=1 work
= 15 days to complete whole work
Q10 | Ex-11 | Cl-8 | Schand Composite Mathematics | Time and Work | Myhelper
Question 10
Sonia can copy 50 pages in 10 hours . Priya and Sonia can copy 300 pages in 40 hours . In how much time can Priya copy 30 pages ?
Sol :
⇒Sonia in 10 hours can copy 50 pages
∴In 1 hour sonia can copy 5010=51 pages
⇒Priya and Sonia together in 40 hours can copy 300 pages
∴Priya and Sonia together in 1 hours can copy 30040=304 pages
⇒Priya's 1 hour work = Priya and Sonia together in 1 hours work - Sonia's 1 hour work
=304−51
=30−204=104=52
⇒Number of hours required to complete the work =Work to be doneone day work
=3052
=5×25
= 12 hours
Q11 | Ex-11 | Cl-8 | Schand Composite Mathematics | Time and Work | Myhelper
Question 11
A tap can fill a cistern in 15 minutes and another can empty it in 18 minutes . Find in how many minutes the tank will be filled up ?
Sol :
⇒Tank filled by tap in one hour =115 part
⇒Tank empty by tap in one hour =118 part
⇒Tank filled in one hour when both taps are open =115−118
=6−590=190
= 90 mins the tank will be filled up
Q12 | Ex-11 | Cl-8 | Schand Composite Mathematics | Time and Work | Myhelper
Question 12
Two taps can fill a cistern in 15 hours and 20 hours respectively . A third tap can empty it in 30 hours . How long will they take to fill the cistern if all the taps are opened ?
Sol :
⇒Tank filled by first tap in one hour =115 part
⇒Tank filled by second tap in one hour =120 part
⇒Tank empty by third tap in one hour =130 part
⇒Tank filled in one hour when all three taps are open =115+120−130
=4+3−260=560=112
= 12 hours take the tank to fill up
Q13 | Ex-11 | Cl-8 | Schand Composite Mathematics | Time and Work | Myhelper
Question 13
Two taps running together can fill a bath in 4 minutes , which is filled by one of the taps by itself in 7 minutes . How long would it take if the other pipe is running by itself ?
Sol :
⇒Bathtub filled by (1st + 2nd) taps in one hour =14 part
⇒Bathtub filled by first tap in one hour =17 part
⇒Bathtub filled by (1st + 2nd) taps in one hour = time taken by 1st tap in one hour + time taken by 2nd tap in one hour
⇒14=17+x
⇒14−17=x
⇒7−428=x
⇒x=328
Time taken by Second tap =283=913 minutes
Q14 | Ex-11 | Cl-8 | Schand Composite Mathematics | Time and Work | Myhelper
Question 14
A cistern can be filled by 3 taps A, B and C when turned on separately in 12 min , 10 min and 15 min respectively . If all are turned on together for 223 minutes and if B and C are then turned off , how much time will A alone take to fill the cistern ?
Sol :
⇒Time taken when all 3 taps are open for a unit time = Tap A open for unit time + Tap B open for unit time + Tap C open for unit time
=112+110+115
=5+6+460=1560=14
A.T.Q
⇒All are turned for 223 minutes =14×223 =14×83=23
⇒Remaining cistern to be filled =1−23 =3−23=13
⇒Tap A alone fill the remaining cistern [1/3 of 12min] =13×12
= 4 minutes
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