A hot-air balloon, headed due east at an average speed of 15 mph and at a constant altitude of 100 feet, passes over an intersection. Find an expression for its distance d (in feet) from the intersection t seconds later.

Do I use D = rt?

If so, then D = 100 feet, right?

Then r = 15 mph, right?

I got:

100 = 15(t)

100/15 = t

Am I right or wrong?

If wrong, what did I do wrong?