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 timetotal distance=r1d+r2d2d=d(r11+r21)2d=r1r2r1+r22=r1+r22r1r2
x=30+402⋅30⋅40=702400=7240≈34.3 mph
Answer: B.