Completing Addition Statements up to 100

An addition statement is any equation like \( 24 + 34 = 58 \). Sometimes one part of the statement is missing, and it is shown as a blank, a box, or a question mark. For example:

\( 24 + \square = 58 \)

\( \square + 37 = 82 \)

Completing an addition statement up to 100 means figuring out what number belongs in that missing spot so the equation is true. This builds directly on skills like adding digits up to 100, since you still need to add and check totals accurately, but now you are working backward from the total to find a hidden part instead of adding two known parts.

Addition and subtraction are inverse operations, which means one can undo the other. If you know \( a + b = c \), then you also know \( c - a = b \). This is the most reliable way to find a missing addend.

Example: \( 24 + \square = 58 \)

Since you already know the total (58) and one addend (24), subtract to find the missing addend: \( 58 - 24 = 34 \). So \( 24 + 34 = 58 \).

Always check your answer by adding the two addends together. \( 24 + 34 = 58 \), which matches the total in the original statement, so the missing number is correct.

Counting on works well when the numbers are close together or when you want a visual way to see the gap between them. Start at the known addend and count up to the total, keeping track of how many steps you took.

This jump from 24 to 58 takes 34 counts, so the missing addend is 34, matching the answer found by subtraction. If counting on by ones feels slow, you can jump by tens first and then by ones, which connects to strategies used when using counting to add digits up to 100.

Some addition statements hide only one digit, not a whole addend. For example:

\( 4\square + 18 = 65 \)

Here the tens digit of the first number is known (4 tens), but the ones digit is missing. Think about the number in terms of tens and ones. The first number is \( 40 + \square \), so the statement becomes:

\( 40 + \square + 18 = 65 \)

Combine the known parts first: \( 40 + 18 = 58 \). Now the statement is simpler: \( 58 + \square = 65 \). Subtract to finish: \( 65 - 58 = 7 \). So the missing digit is 7, and the full statement is \( 47 + 18 = 65 \).

This place value thinking, splitting numbers into tens and ones before solving, is one of many useful addition strategies for working with numbers up to 100.

Complete the statement: \( \square + 45 = 79 \)

Subtract the known addend from the total: \( 79 - 45 = 34 \). So the missing number is 34, and \( 34 + 45 = 79 \).

Complete the statement: \( 36 + 42 = 7\square \)

Add the two known numbers first: \( 36 + 42 = 78 \). The total is 78, so the missing digit in the ones place is 8.

Line up tens and ones carefully, since a missing digit problem is really a place value problem in disguise. Whenever possible, solve the same statement with two different methods, such as subtraction and counting on, to double-check your answer. If a statement mixes several missing digits, solve the ones place first, then the tens place, working the same way you would with regular column addition.

Related lessons

• Math 2