Uniform acceleration

A train accelerates uniformly from rest to a speed $v$ m/s. It continues at this speed for a period of time and then decelerates uniformly to rest. In travelling a total distance $d$ m the train accelerates through a distance $pd$ m and decelerates through a distance $qd$ m, where $p < 1$ and $q < 1$ .

If the average speed of the train for the whole journey is $\dfrac{v}{p+q+b}$ , find $b$ .