What is the rule to this x 1 2 10 y 3 7 39

What are “like terms”? why can we only add like terms?

The administrator of a large assisted living facility wanted to know the average age of the residents living at the facility. he randomly selected 12 residents and determined their age, listed here: 80, 65, 75, 83, 68, 73, 88, 79, 94, 72, 79, 68 what's the average age of the sample of residents? a. 79 years old b. 68 years old c. 75 years old d. 77 years old

I’m still confused on how to work the percentages