You drive 8 miles from your house to your friend's house. your house is at an elevation of 240 feet. your friend's house is at an elevation of 680 feet. what is the mean change in elevation per mile?

First, you would create an equation for this problem. 
(680 - 240) ÷ 8 mi 

Now, all you would have to do is solve it. Using the order of operations (PEMDAS), you would start in the parenthesis. 
(680 - 240) ÷ 8 mi
440 ÷ 8 mi = 55 ft

Your evaluation would increase 55 feet per mile. 

I hope this helps!