Braking distance changes with the square of the change in speed.

E.g.:

If the speed is doubled, braking distance is quadrupled.

If the speed is tripled, braking distance is multiplied by nine.

If the speed is halved, braking distance is reduced to one fourth. If the speed is halved, braking distance is reduced to one fourth.

In this question the speed is multiplied by 4, meaning the braking distance is multiplied by 16 (because 4 x 4 = 16). In this question the speed is multiplied by 4, meaning the braking distance is multiplied by 16 (because 4 x 4 = 16).

You can see this in the diagram below, where the braking distance increases from 2 metres to 32 metres when the speed increases from 20 km/h to 80 km/h. You can see this in the diagram below, where the braking distance increases from 2 metres to 32 metres when the speed increases from 20 km/h to 80 km/h.