If you see an object for the first time, there are several facts you could probably figure out right away, like its color, shape, or size. In the same way, when you see a number family, there are four facts that it gives you right away.
Complete number families tell you four facts about the numbers in them. When you understand all four facts, they will help you solve any problems you may encounter.
In addition and subtraction number families, we know that we have two lesser (smaller) numbers and a greatest (larger) number. We know that we must add the lesser numbers to get the greatest number. We also know that we subtract a lesser number from the greatest number in order to find the other lesser number. We can use this information to help us find a missing number in a number family.
If you have the number family containing the numbers 1, 7, and 8, you know that 1 and 7 are the lesser numbers and 8 is the greatest number. To get the greatest number, add the lesser numbers: 1 + 7 = 8. This is one of the addition facts for this number family. You find the other addition fact by switching the two lesser numbers. The other addition fact is 7 + 1 = 8.
Subtraction facts start with the greatest number. One of the subtraction facts is 8 - 7 = 1. To get the other subtraction fact, you again switch the lesser numbers. The other subtraction fact is 8 - 1 = 7. Once you understand how to find all four facts, you can be sure that you can find any missing number in a number family.
