Let's review what we know about number families. Number families tell us how two or three numbers are related to each other through addition and subtraction. There are two lesser numbers and one greatest number in every number family. You can use addition facts and subtraction facts to find each of these numbers, even if one of them is missing.
If you are given a number family that has 3, 4, and 7 in it, you are able to find out four facts about those numbers. Two of the facts are addition, and two of the facts are subtraction.
To find the addition facts, you add the lesser numbers to find the greatest number. In this case, the addition facts are 3 + 4 = 7 and 4 + 3 = 7. Remember to switch the lesser numbers to find the second addition fact.
To find the subtraction fact, you start with the greatest number and then subtract one of the lesser numbers. The subtraction facts for this number family are 7 - 4 = 3 and 7 - 3 = 4. As with addition facts, you switch the lesser numbers in a subtraction fact to find the other subtraction fact.
