A tank can be filled by two taps X and Y in 5 hours and 10 hours respectively while another tap Z em...........

Quantitative Aptitiude (CLAT) Pipes&cistern

A tank can be filled by two taps X and Y in 5 hours and 10 hours respectively while another tap Z empties the tank in 20 hours. In how many hours can the tank be filled if all 3 taps are kept open?

Explanation:

The easiest way to solve such problems is when we internalise fraction percentage conversions. X can fill in 5 hours.
In 1 hour it can fill 1⁄5 = 20%
Y can fill in 10 hours.
In 1 hour it can fill 1/10 = 10%
Z can empty in 20 hours
In 1 hour it can empty 1/20 = 5% Hence the total efficiency in 1 hour = 20 + 10 - 5
= 25 %
25 % = 1⁄4
Therefore when all 3 tanks are open, the tank can be filled in 4 hours.