Mean (Average) & Missing Values

How the lesson starts: Mean (Average) & Missing Values

The Bake Sale
Four friends baked cookies for the school bake sale. Ava baked 8. Ben baked 10. Cleo baked 12. Dev baked 14. They want to share so everyone has the same number. How many would each friend get?
Even it out
Here are the cookie bars: 8, 10, 12 and 14. Move cookies from the tall bars to the short bars until they're level. They all end up at 11! That level is the mean, or average. Fast way: 8 + 10 + 12 + 14 = 44, and 44 ÷ 4 = 11.
The mystery day
The sale ran 5 days and averaged 20 cookies a day. So 5 × 20 = 100 cookies in all. Monday to Thursday sold 19, 14, 25 and 18: that's 76. So Friday sold 100 − 76 = 24! Watch out: multiply the mean by how many first.