The average of the 5 numbers 2, 7, 9, 10 and x is 5. Find the value of x.

Correct answer: −3. Total = 5 × 5 = 25. The known numbers add up to 28, so x = 25 − 28 = −3.

Background: Average = sum of numbers ÷ how many numbers.

• 3: x = 3 gives a total of 31, not 25.
• 0: x = 0 gives a total of 28, not 25.
• 25: x = 25 gives a total of 53, not 25.

Did you know? A missing number can be negative.

Memory trick: Missing = (average × count) − known sum.