Unit 1, Lesson 3
Prime and Composite Numbers
Grade 4
Activity 1
Your teacher will give you a set of cards that show rectangles.
- Sort the cards into categories in a way that makes sense to you. Be ready to explain the meaning of your categories.
- Group the cards into rectangles that have the same area. Be ready to explain your reasoning.
-
For each group of cards that have the same area, think of at least one more rectangle. Record its length and width. Be prepared to explain your reasoning.
Student Section Summary
We used our understanding of the area of rectangles to learn about factors, multiples, factor pairs, prime numbers, and composite numbers.
If we know the side length of a rectangle, we can find the areas that the rectangle could have. For instance, a rectangle with a side length of 3 could have an area of 3, 6, 9, 12, 15, or other numbers that result from multiplying 3 by a whole number. We call these numbers multiples of 3.
If we know the area of a rectangle, we can find the side lengths that it could have. For example, a rectangle with an area of 24 square units could have side lengths of 1 and 24, 2 and 12, 3 and 8, or 4 and 6. We call these pairs of side lengths the factor pairs of 24.
We also learned that a number that has only one factor pair—1 and the number itself—is called a prime number. For instance, 5 is prime because its only factor pair is 1 and 5.
A number that has two or more factor pairs is a composite number. For instance, 15 is composite because its factor pairs are 1 and 15, and 3 and 5.