Finding Missing Numbers in Two-Digit Equations

Sometimes in math, a number goes missing! You might see an equation with a blank space, like $\_\_ + 34 = 71$ or $85 - \_\_ = 39$.

To find the missing number, we can use the special relationship between addition and subtraction. Think of them as a team: if you know two numbers in the family, you can always find the third!

Missing Number in Addition

Example: $\_\_ + 34 = 71$

In addition, you put two parts together to make a whole. Here, we are missing a part. We know the other part ($34$) and the whole ($71$).

To find a missing part, subtract the part you know from the whole: $71 - 34 = \_\_$ $71 - 34 = 37$

So, the missing number is $37$. Check your work: $37 + 34 = 71$.

Missing Number in Subtraction (Missing Part)

Example: $85 - \_\_ = 39$

In subtraction, you start with the whole and take away a part to find the other part. Here, we know the whole ($85$) and the part that is left ($39$).

To find the part that was taken away, subtract the part left from the whole: $85 - 39 = \_\_$ $85 - 39 = 46$

So, the missing number is $46$. Check your work: $85 - 46 = 39$.

Missing Number in Subtraction (Missing Whole)

Example: $\_\_ - 27 = 48$

Here, the missing number is at the very beginning. This is the whole number we started with before taking $27$ away.

To find the big starting number, add the two parts together: $48 + 27 = \_\_$ $48 + 27 = 75$

So, the missing number is $75$. Check your work: $75 - 27 = 48$.

Quick Rule to Remember

• Missing a part? Subtract!
• Missing the whole (the starting number in subtraction)? Add!