Question

A swimming pool holds 540,000 liters of water. The pool has two drain pipes. When the pool is completely full, the first pipe alone can empty it in 180 minutes, and the second pipe alone can empty it in 120 minutes. When both are draining together, how long does it take to empty the pool?

Answer 1

Answer: 72 minutes

Explanation:

Pipe A can empty the water in 180 minutes = (180/60) =3 hours

Pipe A does 1/ 3 of the job in 1 hour.

Pipe B can empty the water in 120 minutes = (120/60) = 2 hours.

Pipe B does 1/ 2 of thejob in 1 hour.

Together they will do:

1/ 3 + 1/ 2 = 5/6

Together, both pipes will do 5/ 6 of the job in one hour.

To get the total time to completely drain the water will be:

= 60 ÷ 5/6

= 60 × 6/5

= 12 × 6

= 72 minutes

It takes 72 minutes to empty the pool.