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Pipes And Cistern

Practice as much as you can. 20% of the Quantitative Aptitude section is made of this chapter alone. The best approach is to target these questions early. They can be solved quickly and the concepts are easy to understand. Make sure to practice quick mental calculations while solving these questions because it is easy to save time here. Use the short tricks methods of doing the faster calculation.

In such questions, basically a tank has to be filled by two (or more) pipes and we are given:
1) Time taken by each pipe to fill the tank.
2) Total time taken to fill the tank

We’re, usually, given either of the aforementioned information & we’ve to find out the other.
The problem with maximum difficulty asked for Pipes & Cistern can be described in the image below:


Here, we’ve a ‘Tank’ around which the whole question revolves. Basically, we’ve to find out in how long the whole tank could be filled or emptied. Then there are Inlet Pipes (A and B),there can any number of Inlet pipes.

Inlet pipes are responsible for filling the tank. They, basically, bring the water in. The workdone by them is positive.

Then we an Outlet pipe, there can be any number of outlet pipes too. Outlet pipes are responsible for emptying the tank. They, basically, put the water out. The work done by them is negative.


Important formulae, tricks and shortcuts of Pipes And Cistern:

Rules for solving such questions:
1. If a pipe can fill the tank in ‘x ’ hours then, the part filled in 1 hour = 1/x

2. If a pipe can empty the tank in ‘ y’ hours then, the part emptied in 1 hour = 1/y

3. If a pipe can fill the tank in ‘x ’ hours  and another can empty it in ‘y’ hours then, the net part filled in 1 hour = 1/x – 1/y ; Total time taken to fill such tank = xy/y-x

4. A pipe can fill the tank in ‘x’ hrs. Due to leak it is filled in ‘y’ hrs, time taken by leak to empty the tank =xy/y – x hrs
5. If leak time > Inlet pipe then tank will be filled; If leak time < Inlet pipe then tank will be emptied.


For solved problems on above formulas please visit below sections: