Question 120000
In words:
(1 pool / 4 hrs) - (1 pool / 8 hrs) = 1 pool / x hrs
The rate of filling the pool minus the rate of emptying the pool 
equals the rate of filling the pool
{{{1/4 - 1/8 = 1/x}}}
{{{1/8 = 1/x}}}
{{{x = 8}}}hrs
It will take 8 hrs to fill the pool