The Velocity $v$ Of Blood That Flows in a Blood Vessel with Radius

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 $0 \leq b \leq B$ .