The Speed Trap Your Brain Sets

You are driving to a destination 60 miles away at 30 mph. After the first 30 miles, you realize you are behind schedule. How fast must you drive the remaining 30 miles to average 60 mph for the whole trip?

Most people say 90 mph. The correct answer: it is impossible. At 30 mph, you already spent 1 hour covering the first 30 miles. To average 60 mph over 60 miles, the total trip must take exactly 1 hour — which is already used up. You would need to teleport.

This puzzle (sometimes called the "two-speed paradox") reveals that average speed is not the average of two speeds. It is total distance divided by total time.