Introduction
In this activity, we are going to be using addition and subtraction to find missing numbers in number sentences.
We sometimes use the inverse operation (opposite) to find the missing number.
Example 1
Find the missing number in the calculation:
245 + ? = 453
To solve this addition problem, we can use subtraction.
We know that the answer is 453 and we know that something is added to 245 to get to this answer.
We can use this information to find the missing number.
We simply subtract 245 from 453
453 – 245 = 208
We can check that our answer is correct by putting 208 into the original question.
245 + 208 = 453
Example 2
56 + 34 + ? = 280
In this question, we have three numbers to add but are only given two of them.
To solve this addition problem, we can use subtraction.
We know that the answer is 280 and we know that something is added to 56 and 34 to get to this answer.
We can use this information to find the missing number.
First, we do 56 + 34 = 90
Now, we subtract 90 from 280
280 – 90 = 190
We can check that our answer is correct by putting 190 into the original question.
56 + 34 + 190 = 280
90 + 190 = 280
Example 3
Find the missing number in the subtraction.
643 – ? = 255
To solve this subtraction problem, we can use subtraction.
We know that we need to take something away from 643 to get an answer of 255.
We can use this information to find the missing number.
We can simply subtract 255 from 643
643 – 255 = 388
We can check that our answer is correct by putting 388 into the original question.
643 – 388 = 255
Now, it’s your turn to solve some similar problems!
