Fact families

Watch this slideshow all about creating a fact family.

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Fact families are made from three numbers. The numbers in this fact family are 18, 3 and 15. 18 is our whole, 15 represents the red counters and 3 represents the yellow counters.
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If you put them in a number sentence, they form the addition 15 + 3 = 18. The whole here comes at the end of the number sentence because the number sentence is an addition.
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15 and 3, our two parts, can be swapped around and the number sentence is still correct. 3 + 15 = 18.
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What happens if we do a subtraction with the same numbers?
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18 - 15 = 3 is the subtraction. You can't swap the numbers around in a subtraction, so you couldn't write 15 - 18 = 3 as this doesn't number sentence doesn't make sense.
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There are 8 number sentences for each fact family. All of them contain the same numbers.

Part whole models

A part whole model can be helpful when working out simple calculations. They are a visual representation of writing out number sentences and can help solve missing fact family numbers.

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Here is a different fact family. In this case, there are 8 red counters and 7 yellow counters, so the three numbers in this fact family are 8, 7 and 15.
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A part whole model is a good way of representing a fact family. Here the parts are 8 and 7 and the whole number is 15.
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You can cover any one of the three parts that make up the part whole model and use the remaining numbers to find the missing part. Here the missing number is 15 because the two parts added together to make the whole are 8 + 7 = 15.
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It also works if you cover a different part. Here the whole number is 15 and we know that 7 is one of the parts. 15 - 7 = 8.
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A bar model is another way of representing the fact family. The whole is 15 and the two parts 8 and 7 fit under the whole.