A fish tank is being drained through an outlet at the bottom of the tank, and filled by a hose at the top. If the hose can fill the tank in 2 hours and the outlet can empty the tank in 3 hours, how many hours would it take to fill the tank if the outlet were left open?

Solution:

Time:x

1/x = 1/2 - 1/3

x=6

So it takes 6 hours to fill the tank if the outlet were left open