[Brainteaser] Driving on a Track at Different Speeds. Speed of 2nd Lap to Average x mph

Job Interview

You are driving two miles on a one-mile track. You do one lap at 30 miles per hour. How fast do you need to go to average 60 miles an hour?


Interview Questions


Answer: Don’t be inclined to guess 90, because the average of 90 and 30 mph is 60 mph. You completed half your goal by going 30 mph. So that first mile took you one-30th of an hour or two minutes. However, if you averaged 60 mph for two miles, then that should take two-60th of an hour or two minutes to drive two miles. You already drove for two minutes on that first lap, so it’s impossible to average 60 mph. This was a trick question.