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### Example of finding the average speed

Analysis to this problem: there are two constant speeds from city A to city B, so we divide the distance from city A to city B into two parts. Each part has a constant speed. When draw a figure about this problem, we let city A be the point A, let city B be the point B, the place which is 25 miles from the city A is the point P. So, the distance from point A to P is S1 which is 25 miles and the speed in this distance is a constant value of 20 miles per hour. The distant from point P to point B is S2 which is 100 miles. The speed in this distant is a constant value of 60 miles per hour.

Rule applied to solve this problem: when a car with a constant speed drive from point A to point P, the distance from point A to P is S1, the speed that car drive in the distance S1 is V1, and the time spend in driving the distance S1 is t1, then they have the relationship: S1 = V1 × t1 that is, the distance is equal to the constant speed multiply the time spend in this distance. In other word, the time spend from points A to P is equal to the distance from A to P divide by the constant speed from A to P. The average speed is equal to the total distance divide by the total time. The total distance is equal to S1 + S2 and the total time is equal to t1 + t2.