# Lesson Video: Checking Two-Digit Subtraction Problems by Adding Mathematics • 2nd Grade

In this video, we will learn how to use addition to check subtraction and identify the related addition equation that can be solved to check the answer.

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### Video Transcript

Checking Two-Digit Subtraction Problems by Adding

In this video, we will learn how to check two-digit subtraction problems by adding.

This is a two-digit subtraction problem because each of the numbers in the subtraction problem has two digits, a tens digit and a ones digit. To solve this problem, we need to start by subtracting in the ones column. We have seven ones, and we need to subtract four. Seven take away four is three, and seven 10s take away five 10s leaves us with two 10s. 77 subtract 54 equals 23.

We can check this calculation using addition. We know that 77 was the whole amount that we started with. First, we subtracted 54. So, this is one part of 77. We’re left with 23. So, if 77 is the whole, 54 and 23 are the two parts. So, if we add these two parts together, they should give us the whole amount, 77. Let’s start by adding in the ones column. Four plus three equals seven. Five 10s and two 10s gives us a total of seven 10s. 54 plus 23 equals 77.

We checked our subtraction calculation using addition. We checked that 77 take away 54 equals 23 by adding together our two parts, 54 and 23. And we were correct because our addition totals 77. Let’s try some practice questions now. Let’s check our answers to subtraction problems using addition.

You know that 37 subtract 24 equals 13. Is it true that 13 plus 24 equals 37?

In this question, we’re told that 37 take away 24 equals 13. 37 is the whole amount that we start with. We subtract 24, and we’re left with 13. 37 is the whole amount, and the two parts are 24 and 13. We’re asked if it’s true that 13 plus 24 equals 37. What do you notice about these three numbers? We have the same three numbers in our addition calculation, as we do in our subtraction calculation. 37 is the whole amount that we started with. We subtracted 24, and we were left with 13. So, 13 plus 24 equals 37.

We can quickly check this calculation to make sure that it’s true. Let’s start by adding the ones. Three ones and four ones makes seven ones. We know that’s correct. Let’s just add in the tens column. One 10 plus two 10s equals three 10s. It is true that 13 plus 24 equals 37.

What is the number that can fill in the empty places? 37 take away 19 equals what. What plus 19 equals 37.

Let’s start by calculating 37 take away 19. Let’s start in the ones place. We have seven ones, and we need to take away nine. We don’t have enough ones. We’ve taken a 10 and added it to the ones column. Now, we have 17 ones take away nine ones, which is eight. And in the tens column, we have two 10s and we need to subtract one. Two take away one is one. So, we know that 37 subtract 19 equals 18. So, the missing number in our addition calculation must be 18. If the whole amount is 37 — this is the number we started with — we know the missing part is 18 and the other part which we were already given is 19. If the whole amount is 37 and the two parts are 18 and 19, we know that 18 plus 19 equals 37. The number that can fill in the empty places is number 18. 37 subtract 19 is 18. 18 plus 19 equals 37.

There are 58 trees in the street. 14 trees are being cut down. How many trees will be left in the street? Hint: After you subtract, check your answer by adding.

We’re being told to subtract and then check our answer by adding. Let’s start by subtracting. We know that there are 58 trees in the street. And we already know we have to subtract. We need to subtract 14 because 14 trees are going to be cut down. Let’s start by subtracting in the ones column. We’ve got eight ones and we need to subtract four. Eight take away four is four. Now, let’s subtract in the tens column. Five 10s take away one 10 is also four. So, we know that there will be 44 trees left in the street.

Now, we have to check our answer by adding. We know the whole amount or the amount of trees that we started with was 58. We also know that 14 trees were being cut down, and there were 44 trees left. So, we can check our subtraction by adding together the two parts, 14 and 44. And we’re checking to see if they make the whole amount, which is 58. So, let’s check our answer by adding 14 and 44. Four ones plus four ones is eight. One 10 and four 10s is five. 14 plus 44 is 58.

We know our subtraction calculation is correct. We’ve checked it by adding. If there are 58 trees in the street, 14 are cut down, there will be 44 trees left. First, we subtracted 14 from 58 to give us our answer 44. And we checked our answer by adding. 14 and 44 is 58. There will be 44 trees left in the street.

A classroom has 35 students. If 13 of them are boys, how many girls are there?

We know the whole amount or the total amount of students is 35. We also know that 13 students are boys. We don’t know how many students are girls. If we subtract the number of boys, which is 13, from the total, which is 35, that will tell us how many girls there are. Let’s start by subtracting in the ones column. Five ones take away three ones is two. Five take away three is two. And in the tens column, we have three 10s and we need to take away one. Three take away one is also two. So, if a classroom has 35 students and 13 of them are boys, 22 are girls.

We solved the problem by subtracting the number of boys from the total number of students, but we can check our answer using addition. So, we need to add together the number of boys, which is 13, and the number of girls, which is 22, and it should total 35 because we know there are 35 students in the class. We know that three ones plus two ones is five. And we know that one 10 plus two 10s is three 10s. 13 plus 22 is 35. If a classroom has 35 students and 13 of them are boys, we know that there are 22 girls.

What have we learned in this video? We’ve learned how to check our answers in subtraction problems by adding.