We see things around us arranged by color, shape, and size-like rows of fruits in a supermarket. Classifying things make it easier for us to count them. We group similar objects together to solve classification word problems. In these problems, we will see how addition or subtraction facts relate to multiplication.
For example:
Claire keeps her CD collection on 2 shelves. Each shelf has 10 CDs. Movie CDs are on one shelf. Music CDs are on the other shelf. How many CDs does Claire have in all?
First, let's classify. What word tells about all the things in this problem? It's "CD."
Then we make a number family. What do we write for the greatest (larger) number? We write A for "all." This is the missing number. What do we write for the lesser (smaller) numbers? We write "movie CDs" and "music CDs." Next, we put 10 in for movie CDs and 10 in for music CDs, since each shelf holds 10 CDs. Do we add or subtract to figure out the missing number? We add and find out that Claire has 20 CDs.
What's the addition fact? It is 10 plus 10 equals 20 (10 + 10 = 20). Let's see its link to multiplication.
There are 2 shelves with 10 CDs per shelf. That's 2 times 10 equals 20 (2 × 10 = 20). We get 20 whether we combine 2 groups of 10 objects or add equal groups of an object together.
New problem: Cliff has green slippers and yellow slippers. He has 12 green slippers and 18 yellow slippers. How many slippers does he have in all?
The word that tells about all the things in this problem is "slippers." Write A for the missing greatest number. Then put 12 in for green slippers and 18 in for yellow slippers. Add these lesser numbers, and we get 30.
Cliff has 30 slippers!
