Arithmetic (Beginner) · Quick Reference

Average Cheat Sheet

Arithmetic (Beginner) · Lesson 13/21

In one line: the average (or mean) summarizes a group of numbers with a single typical value.

Key Ideas

1Average Formula. Add up all the values, then divide by how many values there are.

2Weighted Average. Used when some values count more than others; multiply each value by its weight, sum them, and divide by the total weight.

3Finding a Missing Value. If you know the average and all but one value, multiply the average by the count to find the total, then subtract the known values.

4Effect of Adding a Value. Adding a new value changes the total count and total sum, which shifts the average up or down.

Worked Examples

Find the average of 4, 8, 6, and 10.

The average of 5 numbers is 20. Four of them are 15, 18, 22, and 25. Find the fifth.

A student scores 70, 80, and 90 on three tests. What score is needed on a fourth test to average 85?

100

Formulas

Average = Sum of values / Number of values

Practice Yourself

Find the average of 10, 20, 30.

Find the average of 5, 7, 9, 11.

The average of 3 numbers is 15; two of them are 10 and 12. Find the third.