Brain Teasers
Averages Aren't So Easy
A man is biking on a 40 mile trip. He travels the first half of the trip at a speed of 15 miles per hour. If he wants his average speed to be 20 miles per hour, at what speed should he travel during the second half?
Answer
30 miles per hour.Step 1: If he wants an average speed of 20 miles per hour on a 40 mile trip, then he needs to travel for a total time of 2 hours.
Step 2: Since he travels at a speed of 15 miles per hour during the first half of the trip, it takes him 80 minutes for the first half, so he has 40 minutes left to travel 20 miles.
Step 3: Since he needs to travel 20 miles in 40, simply add one half of 40 (20), to make 60 (an hour), and do the same to the 20 miles he needs to travel to find out how many miles per hour he needs to travel.
Complicated, eh?
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