Analysis to this problem: there are two constant speed 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 S_{1} 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
S_{2} 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 S_{1}, the speed that car drive in the distance S_{1}
is V_{1}, and the time spend in driving the distance S_{1} is t_{1}, then they have the relationship: S_{1} = V_{1} × t_{1}
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 qual 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
S_{1} + S_{2} and the total time is equal to t_{1} + t_{2}.

If you have any question about this problem, you can send your concern to support@mathtestpreparation.com. Thank you!