If you know the mean and how many values there are, you can find the total: total = mean × number of values. The mean of 4 numbers is 12, so they add up to 48.
To find a missing value, subtract the values you know from the total. If the values are ?, 12, 6 and 15, they make 33 without the missing one, so it is 48 − 33 = 15. The same idea finds what you need to score in a test to reach a target mean.
Worked example
The mean of 5 numbers is 16. Four of them are 10, 20, 15 and 18. Find the fifth.
- Total = 5 × 16 = 80.
- 10 + 20 + 15 + 18 = 63, and 80 − 63 = 17.
Answer:17
Key words
- mean
- total ÷ number of values
- total
- all the values added up
- target
- the mean you want to reach
Practise this
New questions every time.
Common Core-referenced: 7.SP.B.4