Suppose JoJo was driving 30 mph at 5 pm and 40 mph at 7 pm. How far did she travel between 5 and 7 pm?
If JoJo was driving 30 mph at the beginning and 40 mph at the end of the time interval, we can pretend she's been going 35 mph the whole time. Then we estimate her distance to be
35(2) = 70 miles.