Hell-o,

I'm primarily looking for alternative solutions, assuming my method wasn't a fluke

The stopping distance of a car is the number of feet that the car travels after the driver starts applying the brakes. The stopping distance of a certain car is directly proportional to the square of the speed of the car, in miles per hour, at the time the brakes are first applied. If the car’s stopping distance for an initial speed of $\displaystyle 20$ miles per hour is $\displaystyle 17$ feet, what is its stopping distance for an initial speed of $\displaystyle 40$ miles per hour?

$\displaystyle \displaystyle \frac{20}{17} = \frac{40}{x}$

$\displaystyle \displaystyle x = \frac{680}{20}$

$\displaystyle \mathrm{x = 34\ feet}$

This answer is incorrect, but I realized that multiplying it by 2 gives the correct answer (68). Is this method a fluke?