Missing values in number sentences using addition and subtraction - Explanation

Twenty-three add something equals 59. How can we find the missing number in this number sentence? We need to find out what number to add to 23 in order to get 59. We can use a hundreds grid to help us. Let’s count up on the grid, from 23 to 59.

The answer is 36. Twenty-three add 36 equals 59! What if we want to find the missing value another way? Well, look again at our hundreds grid. It shows us something that can help us.

We add two smaller numbers to get a larger number – to reach 59, we added 36 to 23. Look what happens if we subtract one of the smaller numbers, 23, from the larger one, 59. We land on 36! To find the missing value in 23 add something equals 59, we can do 59 subtract 23 and see what we land on. Twenty-three add something equals 59 can be rewritten as 59 subtract 23 equals something.

The answer is 36, and this is our missing value. Twenty-three add 36 equals 59. We’ve learned that, to find a missing number in an addition sentence, we can do the opposite, or inverse of addition – subtraction. Let’s try another example. Something add 14 equals 92.

We know that the inverse of addition is... The inverse of addition is subtraction. We also know that, when we change an addition number sentence into a subtraction number sentence, the subtraction sentence starts with… When we change an addition number sentence into a subtraction number sentence, the subtraction sentence starts with the larger number. So, something add 14 equals 92 can be rewritten as what? Something add 14 equals 92 can be rewritten as 92 subtract 14 equals something.

Now all we need to do is complete the subtraction problem! Ninety-two subtract 14 equals 78. So our missing value is 78. Seventy-eight add 14 equals 92. Here’s one more example to try on your own.

Twenty-one add 27 equals 48. Now you know how to find a missing number in a number sentence using adding and subtracting!