Problem: A car travels 120 miles from A to B at 30 miles per hour but returns the same distance at 40 miles per hour. The average speed for the round trip is closest to:
Answer Choices:
A. 33 mph
B. 34 mph
C. 35 mph
D. 36 mph
E. 37 mph
Solution:
If a car travels a distance d at rate r1 and returns the same distance at rate r2, then
average speed = total time total distance =d/r1+d/r22d=r1+r22r1r2;
∴x=702⋅30⋅40=7240∼34mph