Answer
a) $v=17~m/s$
b) $r=0.317~m$
Work Step by Step
a) $Av={dV \over dt}$
$v={1.2~m^3/s \over \pi (0.15~m)^2}=17~m/s$
b) $A={dV/dt \over v}$
$\pi r^2={dV/dt \over v}$
$r=\sqrt{dV/dt \over \pi v}=\sqrt{1.2~m^3/s \over \pi (3.8~m/s)}=0.317~m$
