The Velocity V of Blood That Flows in a Blood $B$

The velocity v of blood that flows in a blood vessel with radius $B$ and length l at a distance $b$ from the central axis is $v(r)=\frac{P}{4 q l}\left(B^{2}-b^{2}\right)$ where P is the pressure difference between the ends of the vessel and q is the viscosity of the blood. Find the average velocity (with respect to r) over the interval $0 \leq b \leq B \text {. }$