An old auto is to drive a stretch of 2 miles, up and down a hill. Because the auto is so old, you cannot drive the first mile (the ascent) faster than 15 miles per hour.
How fast do you have to drive the second mile (going down the hill) in order to get an average speed of 30 miles per hour?
Answer
There is no way to get an average speed of 30 mph (miles per hour).
If your average speed for the 2 miles stretch must be 30 mph, you should do it in 4 minutes. During the ascent, you would have already taken 4 minutes to travel 1 mile at 15 mph.
Since you have taken 4 minutes to go up the hill, you will need to take 0 seconds to travel the 1 mile descent in order to get an average speed of 30 mph. This is not possible.