Prime and Composite Numbers

Lesson Plan - Get It!

Audio:

• Did you know that some numbers can be divided evenly and some cannot?

These numbers are called prime and composite numbers.

• But what exactly are prime and composite numbers?

As you watch Prime Numbers and Composite Numbers, from Math Is Fun, answer these questions on a piece of paper:

• What is a prime number?
• What is a composite number?
• What are factors?

A composite number is a number that can be divided evenly by numbers other than 1 and itself.

A prime number is a number that can be divided evenly only by 1 and itself.

Any whole number greater than 1 is either prime or composite.

Example 1

• Which of the following numbers is prime: 15, 12, 13?

Think:

• Which numbers can be divided up evenly by a number besides 1 and itself?

15 can be divided by 1, 3, 5, and 15.

Since there are other factors of 15 besides 1 and 15, 15 is a composite number. Odd numbers can be composite numbers.

12 can be divided by 1, 2, 3, 4, 6, and 12.

Since there are other factors of 12 besides 1 and 12, 12 is a composite number.

13 can be divided by 1 and 13.

Since no other whole numbers divided into 13 evenly, 13 is a prime number.

Example 2

• Is the number 37 prime or composite? Explain using full sentences.

Think:

• Can 37 be divided up evenly by a number besides 1 and itself?
• Does 37 have factors besides 1 and 37?

It may be helpful to try to divide a number by 2, 3, 5, or 7.

This is just a starting point to help you find a number that would be divided into 37 easily.

Otherwise, try to find a number between 1 and 37 that divides into 37.

It is very helpful to know your multiplication facts and divisibility rules to help you identify prime and composite numbers.

None of those numbers divide into 37, and there are no other numbers that divide into 37.

Solution

• The number 37 is prime because its only factors are 1 and itself.
• The number 37 is prime because it only has two factors: 1 and 37.
• I tried to divide 37 by numbers between 1 and 37 and found no factors except 1 and 37. So, 37 is prime.:

In your own words, define a prime number and a composite number out loud.

In the Got It? section, you will practice identifying prime and composite numbers!