Question

a hang glider flew to an altitude of 10,000 feet. fifteen minutes later, its altitude was 7,000 feet. what was the average change in elevation per minute?

Answer 1

You can use slope formula 
think of it as a graph initially you start at point (0,10,000)
then you move to (15 , 7,000)
10,000 -7,000 =3,000
15 - 0 = 15
so 3,000/15 is...
200 ft per minute
since you are moving down it is a negative slope -200ft per min