Let the capacity of the pool filled with Xh be 1, which can be obtained according to the meaning of the question.

1 hour water inflow is1/a.

Therefore, the water inflow in time x is (1/a) * X.

The displacement of 1 hour is 1/2a.

So the displacement in x time is1/(2a)] * x.

So the equation is

( 1/a)* x-[ 1/(2a)]* x+ 1/2 = 1

The solution is x = a.