Two dogsled teams raced across a 300-mile course in Wyoming. Team A finished the course in 3 fewer hours than did team B. If team A’s average speed was 5 miles per hour greater than that of team B, what was team B’s average speed, in miles per hour ？（）
A. 12
B. 15
C. 18
D. 20
E. 25
参考答案：D
解析：
You could solve this algebraically, or you could just plug the numbers into the statements in the question and see which one fits the facts. If you start from E, 25 mph, then team B will finish the 300-mile course in 12 hours while team A, at 30 mph, will finish the course in 10 hours; this does not meet the 3-hour difference described in the question. If B travels at 20 mph and A travels at 25 mph, then they will finish the course in 15 and 12 hours, respectively, which meets the 3-hour difference. D is the answer.