Dividing Fractions · Superchallenge problem

Hose A alone can fill a pool in $\dfrac{3}{4}$ of an hour. Hose B alone can fill it in $1\tfrac{1}{2}$ hours. The pool starts empty. Hose B is turned on by itself for $\dfrac{1}{2}$ hour, and then hose A is turned on too. How many more minutes does it take to fill the pool?