Prime and composite numbers
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.
In this video, your child will learn to tell prime numbers from composite numbers.
The lesson stops at each question for your child's answer.
Three problems, each a little harder
Each problem is worked step by step. Your child will solve the last one before Bobby shows how.
Example 1
Is 4 prime or composite?
Example 2
Is 23 prime or composite?
Your child's turn
Is 27 prime or composite?
The three key steps
- 1Find the factor pairs.
- 2Only 1 × itself? Prime. Not 1.
- 3More pairs? Composite.
Mistakes to listen for
- 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.
The lesson shows every step, so your child sees why it works. Read our parent's guide to spotting prime numbers →
Is your child ready to practice what they've learned?
Bobby's Math Club offers unlimited practice for free.
Join the clubMore kid-friendly lessons
More 4th grade Problem Solving, with questions your child answers along the way.
Questions parents ask
Does my child need an account to watch?
No. You can watch this on any device at any time. If your child would like to practice spotting prime numbers, and you'd like parent-friendly reports on your child's progress and growth, you can join Bobby's Math Club for unlimited free practice.
What happens when my child answers a question?
A right answer gets a cheer, and the lesson carries on. If your child isn't sure, they can tap "I don't know yet", and Bobby gives the answer and shows another example. Nothing is scored or stored.
Is this the same lesson that's on YouTube?
Yes. Here, the lesson stops at each question and waits for your child to answer. On YouTube, each question stays on screen for a few seconds, and then the lesson plays on.
Where can I learn more?
There's a short, kid-friendly lesson like this one for every Pre-K to 5th grade math skill, all in our video library. Our parent's guide to spotting prime numbers has more on where kids get stuck and how you can help at home. In the club, your child practices at just the right level, about five minutes a day, and you get parent-friendly reports as they grow. Learn more about how it works →
Practice spotting prime numbers
Questions at your child's level, about five minutes a day, on spotting prime numbers and whatever PK-5 skills they need to develop. Unlimited practice, free forever.
Join the club