Well, I failed my driving permit test because of this one question...

If a car is going at 10mi per hour with a stopping distance of 20 feet. If the car was going 20 miles per hour how far would the stopping distance be?

Well here were the choices

20 - knew this was wrong

40 - chose this

6x - I was deciding this between 40 and 60

7x - I considered this way off.

How did I get this wrong?