posted by L .
an inlet pipe can fill a large water tank in 3 hours. an outlet pipe can empty the same tank in 5 hours. given an empty tank, at 8pm, activate both pipes. what time on the clock will it be before the tank is completely filled?
In any given hour,
fraction of tank filled, x = 1/3-1/5 = 2/15
Time to fill = 1/x = 15/2 = 7.5 hours