You want to fill a bathtub by running water from both the hot and cold taps.
The hot tap takes 6 minutes to fill the tub alone and
the cold tap takes 4 minutes to fill alone.
So you turn them both on at the same time
but you are unaware that you left the plug out.
The plug empties the tub in 12 minutes.
How long will it take to fill the tub?
Solve this by:
(1/6 + 1/4 - 1/12) ^ (-1)
= (1/3)^(-1) = 3