Find the missing number: 12 plus something equals 15.
To help us solve this problem, we’ve been given a bar model. This is a way of helping us to understand the problem. We start off with 12, and then we add something, and the total amount is 15. So what do we add? In this bar model, each square represents one. And so we can see that three squares are added to 12 squares to make 15 squares. And so we can say that the part that we add to 12 to make 15 is three. 12 plus three equals 15.