15 plus three plus three equals what?
This calculation contains three numbers to add together: 15, three, and then another three. We can find the answer by starting with the number 15 and then counting on in threes. We’ll add one lot of three and then another lot of three to find the answer. So let’s go back and start counting in threes: 15, 18, 21.
We started with 15. Then, we added three and then another three. Two lots of three is the same as adding six. And if we add six to 15, we make a total of 21.
15 plus three plus three equals 21.