As we learn more about addition and other concepts in math, it's important to remember what we know about number families. Number families help us quickly figure out how numbers are related to each other.
You've learned that addition and subtraction number families include two lesser numbers and one greatest number (also called the smaller and larger numbers). The lesser numbers added together equal the greatest number. This also means that if we subtract one of the lesser numbers from the greatest number, the difference is the other lesser number.
Each number family tells two addition facts and two subtraction facts. The addition facts start with the lesser numbers, and the sum equals the greatest number. Let's look at the number family containing the numbers 2, 3, and 5. The two addition facts are 2 + 3 = 5 and 3 + 2 = 5.
The subtraction facts for a number family start with the greatest number and then either lesser number is subtracted from the greatest number. For this number family, the two subtraction facts are 5 - 3 = 2 and 5 - 2 = 3.
We can use addition and subtraction facts to help us find a missing number in a number family. If the greatest number is missing, we can add the two lesser numbers to find it. If a lesser number is missing, we can subtract the other lesser number from the greatest number to find the missing lesser number.
