Concept 1 · free preview
What is an average?
Understand
An average is the single value that could replace every item without changing the total. It is what each item gets if the total is ‘shared equally’.
Average = sum of items ÷ number of items
Sum = Average × number of items
Example
Average of 12, 15, 21, 28: sum = 76, count = 4 → 76 ÷ 4 = 19.
Rule
Deviation shortcut: assume an easy value, write each number's difference from it, and add the average of the differences. 47, 52, 49, 56 — assume 50 → differences −3, +2, −1, +6 = +4 → average = 50 + 4 ÷ 4 = 51.
Remember
An average always lies between the smallest and the largest value. Drop any option outside that range.
Common mistake
Leaving a 0 out of the count. A 0 is still an item — include it.
Check yourself
Average of 8, 0, 13 and 19?