Math
posted by Lamont .
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 onethird filled and then the drain pipe is accidentally opened, how long will it take to fill the pool?

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.