Remember: a prime number can only be divided evenly by 1 and itself. Here are some quick tricks to rule numbers out before you even start dividing.
2️⃣ The Even Trick
If a number ends in 0, 2, 4, 6, or 8, it's even — which means 2 can divide it. So no even number is prime... except 2 itself, which is the only even prime there is!
Example: 84 ends in 4, so it's even → not prime.
5️⃣ The 5 Trick
If a number ends in 0 or 5, then 5 can divide it evenly. So it's not prime... unless the number is 5 itself.
Example: 135 ends in 5, so 5 divides it → not prime.
3️⃣ The Digit-Sum Trick
Add up all the digits of a number. If that sum can be divided by 3, then the original number can too!
Example: 123 → 1 + 2 + 3 = 6, and 6 can be divided by 3 → so 123 is not prime.
Example: 849 → 8 + 4 + 9 = 21 → 2 + 1 = 3 → divisible by 3 → so 849 is not prime.
(The only exception is the number 3 itself.)
1️⃣ Why 1 Isn't Prime
A prime needs exactly two divisors: 1 and itself. But 1 only has one divisor — itself. So 1 doesn't count as prime (or as not-prime — it's in its own special group)!
🛑 You Don't Have to Check Every Number
When testing if a number is prime by dividing, you only need to try dividing by numbers up to its square root. If none of those divide evenly, it's prime!
Example: to check 97, you only need to try 2, 3, 4, 5, 6, 7, 8, 9 (since 10 × 10 = 100 is already bigger than 97).
🧪 Try It Yourself
Use these tricks on 51, 91, 111, and 113 — then check your answers with the prime checker. Which one surprised you?