Two towers $A$ and $B$, each of height $20 \mathrm{m}$ are situated a distance $200 \mathrm{m}$ apart. A body thrown horizontally from the top of the tower A with a velocity $20 {\mathrm{ms}}^{-1}$ towards the tower $B$ hits the ground at point $P$ and another body thrown horizontally from the top of tower $B$ with a velocity $30 {\mathrm{ms}}^{-1}$ towards the tower A hits the ground at point $Q$. If a car starting from rest from $P$ reaches $Q$ in $10$ seconds, the acceleration of the car is _________ $($Acceleration due to gravity $=10 {\mathrm{ms}}^{-2})$