Question

A swimming pool holds 480,000 liters of water. The pool has two drainage pipes. When the pool is completely full, the first pipe alone can empty it in 160 minutes, and the second pipe alone can empty it in 240 minutes. When both pipes are draining together, how long does it take them to empty the pool?

Answer 1

The answer is 96 min, here is why:

x / 240 + x / 160 = 1 
2x + 3x = 480 
5x = 480 
x = 96 min

(Drainage rate of first pipe = 480000/240 = 2000 l/min 
Drainage rate of second tube = 480000/160 = 3000 l/min 
Combined drainage rate = 5000 l/min 
Time to drain = 480000/5000 = 96 min. )