Question

When two pipes which can fill the tank in 20 hours each are replaced by two pipes, one with half efficiency and other with double efficiency of previous pipes, the difference in time of filling the tank is?
1. 1
2. 2
3. 3
4. 4
5. 5

Answer 1

Correct Answer - Option 2 : 2

Given:-

Time for pipes to fill the tank = 20 hours

 

Concept use:-

1. Basic time and work concept

2. Relation of time with efficiency.

Calculation:-

Time required to fill tank when both identical pipes are used = 20 / 2 = 10 hours.

Time taken by pipe of double efficiency (half time) = 20 / 2 = 10 hours.

Time taken by pipe of half efficiency (double time) = 20 × 2 = 40 hours.

Time to fill the tank = 1 ÷ (1 / 40 + 1 / 10) = 8 hours.

Difference in time = 10 – 8 = 2 hours.