4th Grade Prime and Composite Numbers worksheets

About this worksheet

Prime and composite numbers ask a fourth grader to look past a number's size and think about its factors instead. A prime number, like 47, has exactly two factors: 1 and itself. A composite number, like 51, has at least one more factor beyond that, such as 3 or 17. Children often guess based on habits that do not hold up: they assume every odd number is prime, or they forget that 2 is even and still prime, since 1 and 2 are its only factors. This worksheet asks your child to decide whether a number up to 100 is prime or composite, and then to explain the decision by naming a factor or by ruling every possible factor out.

Some questions give a number and ask for its category; others give the category and ask your child to pick a matching number, or to spot the number that does not belong. A few questions test the reasoning directly, asking what makes a number prime or composite rather than just naming it. If your child gets stuck, have them try dividing the number by 2, 3, 5, and 7 first, since a number under 100 with none of those as factors is prime.

What your child will learn

• Decide whether a number is prime or composite
• Explain why a number is prime using its factors
• Explain why a number is composite using its factors
• Tell prime and composite numbers apart quickly
• Find a factor other than 1 and the number itself