A pipe can fill a cistern in just 4 hours. It took 8 hours to fill the tank after attaching an outgoing pipe to it at the same time. In how much time alone the outgoing pipe can empty the cistern?

Question: A pipe can fill a cistern in just 4 hours. It took 8 hours to fill the tank after attaching an outgoing pipe to it at the same time. In how much time alone the outgoing pipe can empty the cistern?

Solution:
Let,
the outgoing pipe can empty the cistern in X hours.
total fill-up in one hour
= 1/4 - 1/X
= X - 4/4X

ATQ,
4X/ X - 4 = 8
4X = 8X - 32
X = 8 hours