Equality means that the value of the expression on each side of an addition equation is the same, so a missing number can be determined by comparing quantities, using known sums, or applying addition–subtraction relationships. This includes equations with whole numbers within 1,000, such as a missing addend or expressions on both sides of the equal sign, and develops the understanding that the equal sign does not mean “the answer comes next”; more general algebraic equations and non-whole-number cases are beyond this scope.
An equal sign means both sides have the same value. To find a missing number, first find the value of the side that is complete. Then find what number makes the other side equal.
Example:
Add the numbers on the right:
So the equation becomes:
Ask: What number added to 27 makes 36? Use subtraction:
Put in the box:
Check both sides:
Both sides equal , so is the missing number.
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