Adding Three Numbers and Missing Numbers

Method

1. When adding three one-digit numbers, look at all three numbers first before calculating anything, searching for any two that make a number bond to 10 (such as 4 and 6, or 3 and 7).
2. Add that pair first, since making 10 turns the calculation into '10 plus the leftover number', which is far easier to do in your head than adding strictly left to right.
3. If no pair makes exactly 10, look instead for the two biggest numbers to add first, since starting with a bigger total and adding a small number on is usually easier than the reverse.
4. For missing-number problems, first identify which part of the number sentence is unknown: the first number, the second number, or the total.
5. Rewrite the number sentence as a part-part-whole picture (two parts that combine to make a whole), which makes it visually obvious that any one of the three values can be found if the other two are known.
6. Use the inverse operation to find the missing value: if the whole is missing, add the two known parts; if a part is missing, subtract the known part from the whole.
7. Directly correct the 'always add' misconception: check what the box is actually asking for by covering it with a finger and reading the rest of the sentence aloud, which usually makes clear whether addition or subtraction is needed to fill it.

Practice questions

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Free printable worksheet