# SAT Subject Physics Practice Question 268: Answer and Explanation

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**Question: 268**

**39.** During practice, an athlete runs in a straight line from point X to point Y, and then back along the same path from Y to X. If she runs at a constant speed of 3 m/s from X to Y, and then at a constant speed of 6 m/s from Y to X, what is her average speed for the entire run?

A. 3.5 m/s

B. 4 m/s

C. 4.5 m/s

D. 5 m/s

E. 5.5 m/s

**Correct Answer:** B

**Explanation:**

Perhaps the easiest way answer this question is to choose a numerical value for the distance between X and Y. Let’s choose 180 m. Then the time it takes the athlete to run from X to Y is 60 seconds, and the time it takes to run from Y back to X is 30 seconds. Her average speed for the entire run is the *total* distance traveled, 180 m + 180 m = 360 m, divided by the *total* time, 60 s + 30 s = 90 s. This gives an average speed for the entire trip of (360 m)/(90 s) = 4 m/s. (Note: You would get the same answer, 4 m/s, no matter what positive value you chose for the distance between X and Y.)