During another part of its flight the balloon in exercise 10 had a change in elevation of -901 feet. what was the rate of its descent?

We know that in a given part, in a lapus of time T in units of time, for example seconds, the ballon had a change in elevation of -901 feet.

So in the given time T, the ballon moves downwards a distance of -901 feet, so the rate of change is:

A = -901ft/T

if you multiply this by a amount of time in seconds, the result is the distance that the ballon decends in that lapse of time.