# CAT Practice : Pipes cisterns, Work time

You are here: Home  CAT Questionbank   CAT Quant  Arithmetic: Pipes cisterns  Question 1
Some fill pipes and waste pipes amount to nothing. Between them they result in a net zero.

## Fill pipes and drain pipes

Q.1: A drain pipe can drain a tank in 12 hours, and a fill pipe can fill the same tank in 6 hours. A total of n pipes – which include a few fill pipes and the remaining drain pipes – can fill the entire tank in 2 hours. How many of the following values could ‘n’ take?
a)   24               b)   16               c)   33               d)   13               e)   9                 f)   8

1. 3
2. 4
3. 2
4. 1

Choice A. 3

Click to watch video solution
Click to view the explanation as a slide show

## Detailed Solution

Two drain pipes can drain the same volume that one fill pipe fills. This means that a D-D-F combination has to have a net volume effect of 0.

In spite of this, the tank still gets filled. Only the fill pipes can manage to fill the tank. In addition to all the net zero effect pipes, we need three more fill pipes in order to fill the tank in 2 hours.

So, we can have as many D-D-Fs as we want, but we need one F-F-F at the end to ensure that the tank gets filled in 2 hours.

So the number of pipes will be → (D – D - F).......(D – D - F) + (F – F - F).

The number of pipes has to be a multiple of 3. Only options (a), (c) and (e) fit the description. Choice (A).

## Our Online Course, Now on Google Playstore!

### Fully Functional Course on Mobile

All features of the online course, including the classes, discussion board, quizes and more, on a mobile platform.

### Cache Content for Offline Viewing

Download videos onto your mobile so you can learn on the fly, even when the network gets choppy!

## More questions from Pipes Cisterns, Work Time

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.