Hank's speedometer reads 60 miles per hour, but during a road test he finds that he was actually driving 50 miles per hour. What is the percent error in Hank's speedometer?

 The speedometer read Hank's miles per hour as 20% more than the miles per hour that he actually drove.

Explanation: 20% of 50 which is the miles per hour he actually drove is 10 and 50 + 10 = 60 which is the incorrect miles per hour his speedometer read.