CAT Practice : Pipes cisterns, Work time

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Another one from Work and time for more practice.

Pipes Cisterns - Work Time

    Q.9: A and B together can finish a task in 12 days. If A worked half as efficiently as he usually does and B works thrice as efficiently as he usually does, the task gets completed in 9 days. How long would A take to finish the task if he worked independently?
    1. 12 days
    2. 24 days
    3. 27 days
    4. 18 days

 

  • Correct Answer
    Choice D. 18 days

Explanatory Answer

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Detailed Solution

Let A take ‘a’ days to complete the task and B take ‘b’ days to complete the task.

Thus in one day, A will complete (1/a)th of the task.

Similarly in one day, B will complete (1/b)th of the task.

So in one day, if A and B work together they will complete (1/a + 1/b)th of the task.

Given that A and B together take 12 days to complete the task, then in one day A and B together complete (1/12)th of the task.

Thus, 1/a + 1/b = 1/12 ……Eqn. 1

If A worked half as efficiently as he usually does, then A will take twice the time as he usually takes, i.e., 2a days. Thus in one day, A completes (1/2a)th of the task.

Similarly if B worked thrice as efficiently as he usually does, then B will take one-third the time he usually takes, i.e., b/3 days. Thus in one day, B completed (1/(b⁄3))th or (3/b)th of the task.

Thus when both of them work together, they will complete (1/2a + 3/b)th of the task, given that A and B take 9 days to complete the task.

Thus, 1/a + 1/b = 1/12 ……Eqn. 1

1/2a + 3/b = 1/9 …… Eqn. 2

1/2a + 3/b = 1/9 …… Eqn. 2

Solving Equations 1 and 2 for ‘a’ we should get the answer,

From equation (1) we get 12(a + b) = ab

From equation (2), we get 9(b + 6a) = 2ab

Substituting ab as 12(a + b) in equation (2) we get 9b + 54a = 2 x 12 x ( a + b)

9b + 54a = 24a + 24b;

Or, 30a = 15b,

Or, b = 2a

Now, 12(a + b) = ab, or 12 x 3a = 2a2

a = 18 days.

Answer choice (d).

Correct Answer: 18 days



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  6. Pipes Cisterns - Algebra
Mathematicians are creatures of habit. This topic has been called Pipes and Cisterns because someone named it like it long ago. Pipes and Tanks would be much better - at least makes it sound like wartime.