A car travels at 40 ms$^{-1}$ and decelerates uniformly to 25 ms$^{-1}$ over 30 seconds, then continues at the constant speed of 25 ms$^{-1}$ for a further distance of 500 m. Find the total distance travelled by the car.

During the 30 s deceleration, speed falls uniformly from 40 to 25 ms$^{-1}$, so the average speed is $\frac{40+25}{2}=32.5$ ms$^{-1}$ and the distance covered is $32.5\times30=975$ m. Adding the further 500 m travelled at the constant 25 ms$^{-1}$ gives a total of $975+500=1475$ m. Using only the final speed for the whole deceleration phase gives $25\times30=750$ m there, for a total of 1250 m. Reporting only the deceleration-phase distance and omitting the 500 m constant-speed leg gives 975 m. Using only the initial speed for the whole deceleration phase gives $40\times30=1200$ m there, for a total of 1700 m.