Is 21 Prime?
Check if 21 is a prime number. A prime number is only divisible by 1 and itself.
Result:
✗ NO — 21 is NOT a prime number
Step-by-Step Solution
Step 1
What is a Prime Number?
A prime number is a natural number greater than 1 that has exactly two factors: 1 and itself.
To check if 21 is prime, we need to verify that no number from 2 to √21 divides it evenly.
Step 2
Determine Testing Range
Since 21 is odd, we only need to check odd divisors.
We only need to test up to √21 ≈ 4.58, which rounds down to 4.
If 21 has a factor larger than √21, it must also have a corresponding factor smaller than √21.
Step 3
Test Odd Divisors
Testing: 3, 5, 7, ... up to 4
21 ÷ 3 = 7 (no remainder!)
We found that 3 divides 21 evenly!
Step 4
Conclusion
21 = 3 × 7
Since 21 has factors other than 1 and itself, 21 is NOT a prime number.