# Lesson Video: Adding and Subtracting Numbers to 100 Mathematics • 1st Grade

In this video, we will learn how to solve problems by deciding whether to add or subtract and then completing equations by adding and subtracting within 100.

14:20

### Video Transcript

Adding and Subtracting Numbers up to 100

In this video, we will learn how to solve addition and subtraction problems within 100. In this video, we’re going to learn how to solve addition and subtraction problems. When we add, we’re putting two numbers together to find a total. If there are 17 students on a bus and 12 more get on the bus, we can find the total number of students by putting these two totals together or adding. 17 and 12 gives us a total of 29 students on the bus altogether.

We can think of subtracting as taking apart. If the whole amount or the total number of students on the bus is 29 and 17 students get off the bus, we know there are 12 students left. We would write our first problem as an addition equation or an addition sentence. To find the total number of students on the bus, we had to add together the two parts. 17 students got on the bus plus another 12. To find the total, we need to add together the two parts, 17 and 12. The total number of students on the bus is 29. 17 plus 12 equals 29.

To solve the second problem, we had to subtract or take apart. The whole amount that we started with or the number of students is 29. We have to subtract 17 students because 17 students got off the bus, and there are 12 left. 29 subtract 17 equals 12. So when we’re solving addition and subtraction problems, first we need to think about whether we’re adding together or taking apart. If we’re adding together, we write the addition equation to calculate the total or whole amount. And if we’re taking apart, we need to write our subtraction equation to calculate how many are left.

Let’s try solving some addition and subtraction problems using part-part–whole models, either putting together or taking apart, and addition and subtraction equations to calculate the answer.

In a relay race, Daniel ran 24 meters and then Noah ran 57 meters. What is the total distance they ran altogether?

In this question, two children ran a race. We know that Daniel ran 24 meters, and then Noah ran 57 meters. We have to find the total distance they ran altogether. In this question, we’re given two amounts: 24 meters and 57 meters. We need to put these two amounts together or the two parts together to find the whole or the total amount.

To find the total, we need to add together 24 and 57. This is an addition problem. How could we work out 24 and 57? We could start with the largest number, which is 57, and we could count forward mentally in our heads 24. We could start by adding 20 to 57. We know that 20 is two 10s. If we add two 10s to five 10s, we get seven 10s. So 20 more than 57 is 77. We’re adding 24, so we need to count forward another four: 77, 78, 79, 80, 81. We started at 57. We counted forward 20 and four more. 24 plus 57 equals 81.

We could also use a written method to find the answer. We would start by adding the ones. 24 has four ones. 57 has seven ones. Four plus seven is 11. We can write one in the ones place, and we need to place our 10 in the tens column. Now we can add the tens. Two 10s plus five 10s gives us seven, and one more is eight. 24 plus 57 equals 81. The total distance that Daniel and Noah ran altogether is 81 meters.

There were 98 birds in a tree. 36 flew away. How many are left?

In this problem, we start with 98 birds in a tree. We’re told that 36 birds flew away, and we have to work out how many birds are left. Let’s use our part–whole model to help us write our equation. We know there are 98 birds in the tree to begin with, and 36 fly away. This is a subtraction problem. To find out how many birds are left, we have to subtract 36 from 98. What is 98 subtract 36?

To calculate the answer mentally, we could use a number line. We need to start at number 98 and count back 36. 36 has three 10s which are worth 30. Let’s count back 30 from 98. We can start at 98 and count back three 10s. 98, 88, 78, 68. And 36 has six ones, so we need to count back six from the number 68. 67, 66, 65, 64, 63, 62. 98 subtract 36 equals 62. So there are 62 birds left in the tree.

We could also calculate our answer using a written method. We could use column subtraction. Let’s start by subtracting in the ones column. We’ve got eight ones, and we need to subtract six. Eight take away six is two. Now, we can subtract in the tens column. We have nine 10s, and we need to take away three. Nine take away three is six. And we know we’re correct because we got the same answer twice. If there are 98 birds in a tree and 36 fly away, there will be 62 birds left.

There were 31 tomatoes and 43 strawberries growing in Emma’s garden. She picked eight tomatoes and 11 strawberries when they ripened. Find the equation that shows how many strawberries are still growing in her garden. Is it 31 subtract 11 equals 20, 31 subtract eight equals 23, 43 subtract 11 equals 32, 43 subtract 11 equals 31, or 43 subtract eight equals 35?

There’s a lot of information in this word problem. We’re being asked to work out how many strawberries are still growing in Emma’s garden after she picks the ripe ones. Emma had 43 strawberries in her garden. This is the whole amount or the number of strawberries she had to begin with. We’re also told that emma picked 11 strawberries, but we’re not told how many strawberries she has left. Our part–whole model helps us to think about the subtraction equation. We know Emma had 43 strawberries to begin with, and we also know that she picked 11 ripe strawberries.

So to find out how many strawberries she has left in her garden, we need to subtract the 11 that she picked from the 43 she had to begin with. We need to calculate 43 subtract 11. This is a tricky question because two of the equations show 43 subtract 11. Is 43 subtract 11 32 or 31? If we start at 43, we need to count back 11. 43 subtract 10 gives us 33. And because we’re subtracting 11, we need to subtract one more. 10 and one more is 11. One less than 33 is 32. 43 subtract 10 is 33. 33 subtract one is 32. So 43 subtract 11 equals 32. The equation which shows how many strawberries are still growing in Emma’s garden is 43 subtract 11 equals 32.

Olivia and Victoria are making a robot out of blue and yellow blocks. Olivia has counted out 19 blue blocks. Victoria says they have 39 blocks in total. How many yellow blocks do they have?

We know that Olivia and Victoria have 39 blocks in total. We know that Olivia has counted out 19 blue blocks. We need to work out how many of the blocks are yellow. To solve this problem, we need to take apart or subtract. If we subtract 19, which is the number of blue blocks, from 39, which is the total number of blocks, that will tell us how many of the blocks are yellow. So we need to calculate 39 take away 19. We could find the answer quickly using column subtraction. The number 39 has nine ones, and the number 19 has nine ones. So we have nine ones, and we need to take away nine ones. Nine take away nine is zero.

Now we can just subtract the tens. 39 has three 10s. 19 has one 10. We know that three take away one is two, so three 10s take away one 10 leaves us with two 10s. 39 subtract 19 equals 20. We can check our subtraction calculation using addition. We know that there are 19 blue blocks and 20 yellow. Does 19 plus 20 equal 39? Nine ones plus zero ones is nine. One 10 and two more 10s gives us a total of three 10s. 19 plus 20 is 39. We were correct. Victoria and Olivia have 39 blocks in total. If 19 of the blocks are blue, then 20 are yellow. 39 subtract 19 equals 20. Olivia and Victoria had 20 yellow blocks.

What have we learned in this video? We’ve learned to solve addition and subtraction word problems using part–whole models. And we’ve also learned how to use part–whole models to help us write addition or subtraction equations.
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