A swimmer's four lap times in seconds are 7, 9, 11 and 13. What is the mean lap time?

Explanation

The four times total 7 + 9 + 11 + 13 = 40 seconds, and dividing by the 4 laps gives a mean of 10 seconds. Answering 40 seconds stops at the total and never divides, so it reports how long all four laps took together instead of how long a typical lap took.