每日一题

Mathematics>Standard Multiple Choice

Read the following SAT test question and then click on a button to select your answer.

Lisa gives her little brother Sam a 15-second (sec) head start in a 300-meter (m) race. During the race, Sam runs at an average speed of 5 m/sec and Lisa runs at an average speed of 8m/sec, not including the head start. Which of the following best approximates the number of seconds that had passed when Lisa caught up to Sam?

A.5
B.25
C.40
D.55

答案和解析

答案：B

解析：

Choice B is correct. Let t represent the time, in seconds, that Sam spends running, let d represent the distance run, and let r represent the rate at which Sam is moving.
Then, since d = rt, 5t represents the total distance, in meters, Sam runs in t seconds.
Since Lisa gives her little brother a 15-second head start, it follows that she runs for t-15 seconds at a rate of 8m/sec.
So, 8(t-15) represents the total distance, in meters, Lisa runs in t-15 seconds.