Jeff is standing 400 feet from a hot air balloon.
At the moment that the balloon starts raising (straight up) at 6 feet per second, Jeff backs away from the balloon at 2 feet per second.
How fast is the distance between Jeff and the balloon increasing after 30 seconds?

Set up an equation and use the derivative to solve this problem. Note that both x and y have rates of change that are proportional. (dy/dt = 6 ft per sec and dx/dt = 2 feet per sec, could be assumed at t = 1 second)