Question

It takes an older pump 4 times as long to drain a certain pool as it does a newer pump. Working together, it takes the two pumps 3 hours to drain the pool. How

long will it take the newer pump to drain the pool working alone?
Do not do any rounding.

Answer 1

New pump = 3.75 hours

Old pump = 15 hours

Explanation:

Let :

Time taken by newer pump = x hrs

Time taken by older pump = 4x hrs

Time taken when working together = 3 hours

Drain rate :

Newer pump = 1/x

Older pump = 1/4x

Combined = 1/3

Hence :

Newer pump rate + older pump rate = combined rate

1/x + 1/4x = 1/3

Take the lcm

(4 + 1) / 4x = 1/3

Cross multiply

5 * 3 = 4x

15 = 4x

x = 15/4

x = 3.75

The old pump will take :

4x = 4 * 3.75 = 15 hours