The distance traveled by a falling object(subject only to gravity, with wind resistance ignored)is directly proportional to the square of the time it takes to fall that far. If an object falls 100 feet in 2.5 seconds, how far does it fall in 5 seconds?

MY WORK:

I developed the equation d = k/(t^2) from the given information, where d = distance, k is the constant of variation and t = time in seconds.

After doing the math, I got d = 25 feet, which makes no sense. The right answer is 400 feet. How do I get 400 feet? Is my equation wrong?