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 Time and Work Questions

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FACTS  AND  FORMULAE  FOR  TIME  AND  WORK  QUESTIONS

 

 

1. If A can do a piece of work in n days, then A's 1 day's work =1n

2. If A’s 1 day's work =1n, then A can finish the work in n days.

 

3. A is thrice as good a workman as B, then:

Ratio of work done by A and B = 3 : 1.

Ratio of times taken by A and B to finish a work = 1 : 3.

 

NOTE : 

Efficiency∝1No of time units

 

∴ Efficiency × Time = Constant Work

 

Hence, Required time = WorkEfficiency

 

Whole work is always considered as 1, in terms of fraction and 100% , in terms of percentage.

In general, number of day's or hours = 100Efficiency

Q:

A take twice as much time as B or thrice as much time to finish a piece of work. Working together, they can finish the work in 2 days. B can do the work alone in  ?

A) 8 hrs B) 12 hrs
C) 6 hrs D) 4 hrs
 
Answer & Explanation Answer: C) 6 hrs

Explanation:

Suppose A, B and C take x, x/2 and x/3 respectively to finish the work. 

Then, (1/x + 2/x + 3/x) = 1/2 

=> 6/x = 1/2      => x = 12 

So, B takes 6 hours to finish the work.

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Q:

A certain job was assigned to a group of women to do it in 20 days. But 12 women did not turn up for the job and the remaining did the job in 32 days. The original number of women in the group was?

A) 36 B) 32
C) 22 D) 28
 
Answer & Explanation Answer: B) 32

Explanation:

Let the total women in the group be 'W'

Then according to the given data,

 W x 20 = (W-12) x 32

 => W = 32 

Therefore, the total number of women in the group = 32

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Q:

Two persons Shyam and Rahim can do a job in 32 days together. Rahim can do the same job in 48 days alone. They started working together and after working 8 days Rahim is replaced by a third person Ram whose efficiency is half that of Rahim. Find in how many days the remaining work will be completed by both Shyam and Ram together?

A) 16 days B) 72/5 days
C) 15 days D) 96/5 days
 
Answer & Explanation Answer: B) 72/5 days

Explanation:

Work done by Shyam and Rahim in 8 days = 8/32 = 1/4

Remaining work to be done by Shyam and Ram = 1 - 1/4 = 3/4

Given efficieny of Ram is half of Rahim i.e, as Rahim can do the work in 48 days, Ram can do the work in 24 days.

One day work of Ram and Shyam = (1/32 - 1/48) + 1/24 = 5/96

Hence, the total work can be done by Shyam and Ram together in 96/5 days.

 

Therefore, remaining work 3/4 can be done by them in 3/4 x 96/5 = 72/5 = 14.4 days.

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Q:

In a public bathroom there are n taps 1,2,3...n. Tap1 and Tap2 take equal time to fill the tank while Tap3 takes half the time taken by Tap2 and Tap4 takes half the time taken by Tap3. Similarly each next number of tap takes half the time taken by previous number of Tap i.e, K-th Tap takes half the time taken by (K-1)th Tap. If the 10th tap takes 2 hours to fill the tank alone then what is the ratio of  efficiency of 8th tap and 12th tap, respectively?

A) 4:1 B) 5:3
C) 16:1 D) 1:16
 
Answer & Explanation Answer: D) 1:16

Explanation:

Time taken by 8th tap = 2 x 2 x 2= 8 hours

Time taken by 12th tap = 2x (1/2) x (1/2)  = 1/2 hour

Ratio of time taken by 8th tap and 12th tap = 8 : 1/2 =16:1

Therefore, Ratio of efficiencies of 8th tap and 12th tap =1:16

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Q:

A can do a work in 9 days, B can do a work in 7 days, C can do a work in 5 days. A works on the first day, B works on the second day and C on the third day respectively that is they work on alternate days. When will they finish the work ?

A) [7 + (215/345)] days B) [6 + (11/215)] days
C) [6 + (261/315)] days D) [5 + (112/351)] days
 
Answer & Explanation Answer: C) [6 + (261/315)] days

Explanation:

After day 1, A finishes 1/9 of the work.

After day 2, B finishes 1/7 more of the total work. (1/9) + (1/7) is finished.

After day 3, C finishes 1/5 more of total work. Total finished is 143/315.

So, after day 6, total work finished is 286/315.

 

Now remaining work =  29 /315

 

On day 7, A will work again 

 

Work will be completed on day 7 when A is working. He must finish 29/315 of total remaining work.

 

Since he takes 9 days to finish the total task, he will need 261/315 of the day. 

 

Total days required is 6 + (261/315) days.

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Q:

16 women can do a piece of work in 10 days while 15 men can do the same work in 12 days. All men and all women work alternately starting with all men, then find the time taken by them to complete the whole work?

A) 10 5/2 days B) 11 days
C) 11 2/5 days D) 12 1/2 days
 
Answer & Explanation Answer: A) 10 5/2 days

Explanation:

Let efficiency of every man and every woman be 'm' unit/day and 'w' unit/day respectively

15×12×m = 10×16×w

⇒ m/w = 8/9

Total work = 15 × 12 × 9 = 1620 units

In 2 days, total work done = 15 x 9 + 16 x 8 = 263 units

So, in 10 days work done will be = 263 × 5 = 1315 units

Remaining work will be done in = (1620-1315)/(15×8) = 5/2 days

Total days = 10 5/2 days.

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Q:

Concept of Efficiency

A can do a job in 12 days and B can do the same job in 6 days, in how many days working together they can complete the job?

Answer

A's 1 days work = 1/12 


B's 1 days work =1/6


Therefore, (A+B)'s 1 day's work= 1/12 + 1/6 = 1/4       


Therefore, Time taken by both to finish the whole work = 114 = 4 days


 


Alternatively :


Efficiency of A =100/12 =8.33%


Efficiency of B =100/6=16.66%


Combined efficiency of A and B both = 8.33+16.66=25%


Therefore, Time taken by both to finish the work (working together) =100/25= 4days

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Q:

Application of Inverse Proportion

If 20 persons can do a piece of work in 7 days then calculate the number of persons require to complete the work in 28 days

Answer

Number of persons * days = work  


20 * 7 = 140 man- days 


 


Now,  x * 28  = 140 men- days


=> x =5         


Therefore in second case the required number of person is 5.


 


Second method:


Since work is constant, therefore  M1 x D1= M2 x D2


20 x 7 = M2 x 28


=> M2 = 5 


 

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