A hang glider flew to an altitude of 10000 feet. Fifteen minutes later, it's altitude was 7000 feet. What was the change in elevation per minute?

The speed is the change of distance over the change of time.

s = (d - d1) / (t - t1)

where the subscript 1 indicates as "initials."

The initial time starts at zero. So,

s = (7000 - 10000) / (15 - 0) = - 200 feet (elevation) per minute

The value is negative because the hang glider flew downward.