Find the Missing Number

The missing number does not always come last. It can be anywhere.

What the equal sign means

The equal sign does not mean "the answer comes next". It means both sides are worth the same.

Equal means both sides have exactly the same value.

So 15 = 7 + 8 is a perfectly good sentence. It says the same thing backwards.

Check by putting it back

Once you have an answer, put it in the sentence and see whether both sides match.

Where the unknown can hide

A missing number can sit in three places: at the start (? + 6 = 14), in the middle (7 + ? = 15), or at the end (9 + 4 = ?). Only the last one is ordinary addition. The other two look harder but are the same fact family seen from a different side.

Turn it into a subtraction

For 7 + ? = 15, ask what 15 - 7 is. The whole is 15, one part is 7, so the other part is 8. Whenever the whole is known and one part is missing, subtract. Whenever both parts are known, add. Deciding which case you are in is most of the work.

Always put your answer back in

Once you think ? is 8, write the sentence out with 8 in place of the question mark: 7 + 8 = 15. If it is true, you are done. This check costs almost nothing and catches the most common error, which is subtracting the wrong way round.