At first you might think, “Halfway at 40 mph, halfway at 60 mph—the average
is 50.” But that’s way too simple for an SAT Subject Test: Math 1 word
problem. The average rate would be 50 if Martha drove half the time at 40 and
half the time at 60. But it’s half the distance she drives at 40 and half the
distance she drives at 60. So she actually spends more time driving at the
slower speed, and her average speed will be something closer to 40 than to
- So the answer’s going to be either (A) or (B).
Average speed is defined as total distance divided by total time:
In Example 8 the distance is not specified, so call it d.
AVERAGE RATE FORMULA
To get the total time, think about the time for each half-distance. Martha
travels the first miles at 40 miles per hour: