Question

Two pipes A and B can fill a tank in 5 hours and 20 hours respectively. Both pipes together can fill the same tank in:
1. 4 hours
2. 6 hours
3. 10 hours
4. 2 hours

Answer 1

Correct Answer - Option 1 : 4 hours

CONCEPT USED:

Here the capacity of the tank is constant, the only factor which decides the time difference is the efficiency of the 2 pipes.

CALCULATION:

We know that time taken is inversely proportional to the efficiency of the pipes and the efficiencies get added up when both of the pipes fill the tank together.

The amount of the tank filled by pipe A in 1 hour = 1/5

The amount of the tank filled by pipe B in 1 hour = 1/20

The amount of the tank filled by both pipe A and B together in 1 hour

⇒ \({1 \over 5} + {1 \over 20} = {1 \over 4}\)

∴ time taken to fill the tank= 4 hours

The efficiencies always add up if the number of pipes goes on increasing and not the time.