An inlet pipe on a swimming pool can be used to fill the pool in 24 hours. The drain pipe can be used to empty the pool in 40 hours. If the pool is one-third filled and then the drain pipe is accidentally opened, how long will it take to fill the pool?

Answer 1

If the pool was empty it would take:
1/T = 1/24 - 1/40,
LCM = 120.
1/T = 5/120 - 3/120 = 2/120 = 1/60.
T = 60h if tank was empty.

T = (2/3) * 60 = 40h.