Same setup. Speed of current = ?

The current's speed equals half the difference between the downstream and upstream speeds. With the given distance and times, the downstream speed is $\dfrac{d}{q}$ and the upstream speed is $\dfrac{d}{p}$, so the current's speed is $\dfrac{1}{2}\left(\dfrac{d}{q}-\dfrac{d}{p}\right)=\dfrac{d}{2}\left(\dfrac{1}{q}-\dfrac{1}{p}\right)$.