A car travels from Mayville to Rome at an average speed of 30 miles per hour and returns immediately along the same route at an average speed of 40 miles per hour. Of the following, which is closest to the average speed, in miles per hour, for the round-trip?
(A) 32.0 (B) 33.0 (C) 34.3 (D) 35.5 (E) 36.5
We can solve this one quickly if we assign a "nice" distance between Mayville and Rome ("nice" means a distance that works well with 30 miles per hour AND 40 miles per hour)
So, let's say the distance between Mayville and Rome =
120 miles
Time = distance/speed
So, time from Mayville and Rome (at 30 mph) = 120/30 =
4 hours
Time from Rome to Mayville (at 40 mph) = 120/40 =
3 hours
Average speed = (total distance traveled)/(total time)
= (
120 +
120)/(
4 +
3)
= 240/7
=
34.something (if we use long division here, we'll stop once we get to the ones digit)
=
C
Cheers.
Brent
