Video: Writing and Solving Expressions for Word Problems Requiring the Addition of Three Whole Numbers

Bethani Gasparine

18 students are wearing colored tops for a game. There are 7 red tops, some green tops, and 5 blue tops. Write an addition sentence that matches this story. How many green tops are there?

01:55

Video Transcript

18 students are wearing colored tops for a game. There are seven red tops, some green tops, and five blue tops. Write an addition sentence that matches this story. How many green tops are there?

So we know that there are seven red tops, some green tops — we don’t know how many there are — and we have five blue tops. And altogether, there are 18 students. So we would need to add the number of t-shirts together.

An addition sentence that matches this story would be seven plus question mark plus five equals 18, because we needed to add seven red tops, some green tops that we didn’t know how many they were — so that was a question mark — plus five blue tops. And altogether, there would be 18 tops because there were 18 students.

And now we need to solve for the number of green tops. So this means we need to solve for the green question mark that represents the number of green tops. So the first thing that we need to do on the left-hand side of the equation would be to add the seven and five. And seven plus five is 12.

So now to solve for the question mark, we need to subtract 12 from both sides of the equation. On the left, the 12s cancel. And we just have the question mark left. And on the right-hand side of the equation, 18 minus 12 is six. So there are six green tops.

We also could have solved from this point. 12 plus question mark equals 18. So 12 plus what would give us 18? Well, 12 and then counting 13, 14, 15, 16, 17, 18. We needed six more tops. So once again, the addition sentence that matches the story was seven plus question mark plus five equals 18. And there were six green tops.

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