A car travels twice as fast as a second car. The faster car will take how much more distance to stop?

The stopping distance is not straightforward and depends on two factors:

The time for a driver to react to a situation called the "thinking distance". The distance travelled in between the driver realising he needs to brake and actually braking and during which the car carries on moving.

The distance taken to stop once the brakes are applied called the "braking distance".

Both these factors combine to make the total stopping distance, which is not a linear scale.

Thus going twice as fast does not simply double the distance.

Thus without knowing what the speed of the slowest car is we can not tell you "how much more distance to stop"

at 20 mph the distance is 40 feet (under ideal conditions).

at 30, 75 feet

at 40, 118 feet

at 50, 175 feet

at 60, 240 feet

at 70, 315 feet