Calculus · Hard problem

A reservoir's inflow rate is $r(t)=20-2t$ thousand gallons/hour (it decreases as the reservoir fills and feeds a growing counter-outflow). Its volume above the starting level is $V(t)=\displaystyle\int_0^t(20-2s)\,ds$. Find the time $t>0$ at which $V$ is maximized, and the maximum volume.