Two roommates went by car to a movie theater, which is 20 miles from their apartment. Due to heavy traffic, it took them 40 minutes to get there. What was their average speed in miles per hour?

Forty minutes is \(\frac{\mathrm{40} }{\mathrm{60}}\) or \(\frac{\mathrm{2} }{\mathrm{3}}\) of an hour. Because average speed is distance divided by time, divide 20 miles by \(\frac{\mathrm{2} }{\mathrm{3}}\) hour to obtain 30 mph. It’s worthwhile to note that 60 mph is a mile a minute. In this case, it took 40 minutes to go 20 miles, and this is half a mile a minute, so their average speed was 30 mph.