Spotting prime numbers
In 4th grade, children learn to tell whether a whole number up to 100 is prime, with exactly two factors, or composite, with more than two.
By Robert “Bobby” BarnettMath teacher, parent, and founder of Bobby’s Math Club
Try these with your child
These are real questions from our 4th grade practice. Tap the speaker to hear one read aloud, or the light bulb for a hint, and talk about the answer together.
Question 1 of 3
Mia's locker number is a prime number between 10 and 15. Which could it be?
- 14
- 13
- 15
Why this matters
Spotting a prime number is a small piece of detective work, and it sharpens your child’s sense of how numbers fit together. Primes are the building blocks that every whole number greater than 1 is made from by multiplying. That matters in later grades, when your child puts fractions in simplest form and, in middle school, breaks numbers into their prime factors. It even explains a puzzle at home: 23 kids can’t split into equal teams the way 24 can.
Every prime check starts with factor pairs, so if those are still new, your child can review finding factors and multiples up to 100.
Where kids can get stuck
- Thinking every odd number is prime, so they call 9, 15, or 21 prime even though 9 = 3 × 3.
- Calling 2 composite because it is even, when it is the only even prime number.
- Calling 91 prime because no times-table fact comes to mind, even though 91 = 7 × 13.
How you can help
- Play prime-or-not on walks using house numbers under 100. Your child calls out whether each one is prime or composite, and names a factor pair as proof for each composite number.
- Talk about ages at dinner. Ask whose age in the family is prime this year, and when your child’s next prime birthday will be. Someone who is 41 is in a prime year; at 42, they are not.
- Hunt for primes at bedtime, one decade at a time. Pick the 20s or the 70s and find every prime in it together: 23 and 29 in the 20s; 71, 73, and 79 in the 70s.
For more, Bobby's Math Club has free practice at your child’s level.
Questions parents ask
Why is my child counting factors to decide if a number is prime?
That is what prime means. A prime number has exactly two factors, 1 and itself, and a composite number has more than two. So your child checks: 21 has the factors 1, 3, 7, and 21, which makes it composite. Learning the reason, rather than memorizing a list, means your child can test any number up to 100 and explain the answer.
Is 1 a prime number?
No. A prime number has exactly two different factors: 1 and itself. The number 1 has only one factor, which is 1, so it is neither prime nor composite. Children often guess that 1 is prime because it cannot be broken into smaller factors. It is a good question to talk through, since it shows your child is thinking about the definition carefully.
Is there a quick way to tell if a number up to 100 is prime?
Yes. For any number from 2 to 100, your child only needs to try dividing by 2, 3, 5, and 7. If none of them goes in evenly, the number is prime, and 2, 3, 5, and 7 are prime themselves. So 89 is prime, but 87 is composite, because 87 = 3 × 29. A handy first check: any even number other than 2 is composite.
Make learning math fun
Bobby's Math Club turns 4th grade math practice into something fun to do together.
- Free, unlimited practice for every child in your family
- Practice that starts where your child is and grows with them
- You see their progress and what to work on next
Written by Robert “Bobby” Barnett
Bobby is an award-winning math teacher, the author of Meet Every Learner’s Needs, and the father of two elementary-school math learners. He uses Bobby’s Math Club with his own kids a few times every week. Learn more or contact Bobby.
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Join the clubNote for teachers: This skill is covered by Common Core math standard 4.OA.B.4: “Find all factor pairs for a whole number in the range 1-100. Recognize that a whole number is a multiple of each of its factors. Determine whether a given whole number in the range 1-100 is a multiple of a given one-digit number. Determine whether a given whole number in the range 1-100 is prime or composite.” Learn how Bobby's Math Club can help your students grow. (Common Core 4.OA.4)