Two buses $A$ and $B$ are running over a two line bridge with the same speed $20{ \mathrm{ms}}^{-1}$ in opposite direction. Both buses approaches a car C moving with a speed of $15{ \mathrm{ms}}^{-1}$ in the same direction of bus $A$. To avoid accident the driver of bus A decides to over take car C just before the bus B does, at the instant when buses A and B are $500 \mathrm{m}$ apart and provides at least required acceleration of  $2{ \mathrm{ms}}^{-2}$ to the bus.