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| Math Central | Quandaries & Queries |
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Question from Camasha, a parent: I am trying to help my daughter with fact families. I understand the premise from your previous explanation on the site . Generally when applying addition and subtraction their are two add and two subtraction utilizing all three numbers. However what do you do when you have a series like like 9, 9, 18 |
Hi Camasha,
The first fact family your daughter saw was probably made up of something like 2, 3 and 5, that is two positive numbers and their sum. There are then four members in the family.
- The sum of the two smaller numbers to give the larger number
2 + 3 = 5.
- The sum of the two smaller numbers in the reverse order give the larger number
3 + 2 = 5.
- The largest number minus one of the smaller numbers
5 - 2 = 3.
- The largest number minus the second smaller number
5 - 3 = 2.
If you apply the same rules to 9, 9 and 18 you get
- The sum of the two smaller numbers to give the larger number
9 + 9 = 18.
- The sum of the two smaller numbers in the reverse order
9 + 9 = 18.
- The largest number minus one of the smaller numbers.
18 - 9 = 9.
- The largest number minus the second smaller number.
18 - 9 = 9.
Facts 2 and 4 are redundant so the fact family only has two members
- 9 + 9 = 18
- 18 - 9 = 9
This shouldn't be surprising. There are many kinds of families, average families (a man a woman and two and a half children), extended families, single parent families, blended families, ...
Penny
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