Video: Adding Two-Digit Numbers to One-Digit Numbers Using the Relation between Addition and Subtraction

Use the model to help you find the missing number: 18 + _ = 20 .


Video Transcript

Use the model to help you find the missing number 18 plus something equals 20.

This type of model is called a bar model because it’s made up of two different bars: one on the top and one on the bottom that split into two parts. And we can use the way that the bars and the numbers on them fit together to understand how to answer questions. Let’s have a look at this particular bar model.

Our number sentence says that 18 plus a mystery number equals 20. What does each bar represent? We can see 18 and the unknown number on the lower part of the bar model. These are the two parts that make up the whole. And the top bar shows the whole amount, which is 20. So we know the whole amount and one of the parts. How do we find out the other part?

We know that if we start with a whole amount and then we subtract part, we’re left with the remaining part. And so one way to find the unknown number is to start with the whole and to subtract the part we already know: 20 take away 18. 20 take away 18 equals two.

Another method we could use to find the answer is to see that 20 and 18 are very close together. So we could think about this part here as being the difference between 18 and 20. So we could start with the number 18 and count on: one, two. And so although we thought about our bar model in different ways, we’ve used it to find the same answer: 18 plus two equals 20.

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