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Adding Ones to Three-Digit Numbers

In this video, we will learn how to model adding a one-digit number to a three-digit number and investigate which digits in the number change. 263 is a three-digit number. It has a hundreds digit, which is a two, and that’s worth two 100s. It has a tens digit — it’s a six, and this is worth six 10s or 60 — and a ones digit, which is a three, and it’s worth three ones. In this video, we’re going to be adding ones to three-digit numbers. And we’re going to investigate which digit in the number changes.

Watch what happens to the digits in the number 263 if we add three. We’ve added one, two, three. 263 add three is 266. Did you notice what happened to the digits in the number 263? The hundreds digit didn’t change, and the tens digit didn’t change. Only the ones digit changed. We have three ones. We added three more ones. Three plus three is six. So, the ones digit increased by three.

Using a place value table and place value counters is a really useful way to help us think about what happens to a three-digit number when we add more ones. But we can also add ones to three-digit numbers in our heads. How could we add five to 152 mentally? That means we need to work it out in our head without any place value equipment to help. We need to start at 152 and count on five more. 153, 154, 155, 156, 157. 152 add five equals 157.

Did you notice what happened to the ones digit as we counted on? We started at 152. If we count on one more, we have 153. 154, 155, 156, 157. We counted on five. And each time we counted on one more, the ones digit increased by one.

Another way to add a one-digit number to a three-digit number is to use a number line. To add seven to 682, we would need to start at number 682 and count forward seven. One, two, three, four, five, six, seven. We started at 682 and counted forward seven. 683, 684, 685, 686, 687, 688, and 689.

Did you notice what happened to the ones digit as we counted forward? Each time we added one, the ones digit increased by one. If we partition or break our number apart into an amount of ones and an amount of hundreds and tens, it helps us to see that all we’re doing is adding seven and two. Two ones and seven ones equals nine ones. 680 and nine makes the number 689.

So far, we’ve learned that we can add one-digit numbers to three-digit numbers in lots of different ways. We can count forward using a number line, we can count on in our heads, we can use place value equipment such as place value tables and counters or blocks, and we could also use partitioning or breaking the number apart to help us think about how to add the ones to a three-digit number. Let’s put into practice what we’ve learned now by answering some questions.

Find the result of 374 plus five.

In this question, we’re adding a one-digit number to a three-digit number. One way we could add five to 374 is to count on in our heads. We need to start at 374 and count on five more. 375, 376, 377, 378, 379. We started at 374 and we counted on five more. 374 plus five equals 379.

Use the following number line to find 742 plus two.

In this question, we have to add a one-digit number to a three-digit number. The number 742 has three-digits, and we have to add two which is a one-digit number. We’ve been given a number line to help us add. The first thing we need to do is find the number 742 on the number line. And then we need to count forward two; this is because we’re adding two to 742. So, we’re going to start at 742. Add one, two. We finished on number 744. 742 plus two equals 744. We added our one-digit number to our three-digit number by counting forward on a number line.

Find the total by first adding the ones. 526 plus two equals what.

In this question, we’ve been given a part–whole model to help us break the number 526 apart. This is to make it easier for us to add the ones. The number 526 has six ones. We can break this number apart into an amount of ones and an amount of hundreds and tens. 520 plus six is 526. Now, what we need to do is add our two ones from the number two to our six ones. And we know that six plus two equals eight. Now, we have 520 plus eight. 526 plus two equals 528. We broke our three-digit number apart and first added the ones. Six ones plus two ones gave us eight ones. And 520 plus eight is 528.

Noah has 343 stamps. His friend Hannah gives him five more stamps. How many stamps does Noah have now?

This is an addition problem. Noah has 343 stamps, and Hannah gives him five more. So, to find out how many stamps Noah has all together, we need to add 343 and five. The number 343 is a three-digit number. It has three 100s, four 10s, and three ones. And we have to add five ones. We just need to add the ones because the number five has no hundreds and no tens.

What is three ones plus five ones? We know that three plus five is eight, so 343 plus five equals 348. There was nothing to add to our hundreds digit and nothing to add to our tens digit. We just had to add five more ones to the three ones in our number. We know that five more than three is eight. The only digit which changed was the ones digit. 343 plus five equals 348.

What have we learned in this video? We’ve learned how to add a one-digit number to a three-digit number using place value counters, number lines, and by counting on in our heads.