Step by step
The question
Question: is 21 a prime number?
Is 21 prime?
A prime number has exactly two factors: 1 and itself.
So we look for any other number that goes into 21.
prime: only 1 and itself go in
1 Try dividing
Does 2 go in?
21 divided by 2 is 10, with 1 left over.
So 2 is not a factor.
= 21 ÷ 2 = 10 r 1 ✗
r 1 = 1 left over
Does 3 go in?
21 divided by 3 is 7.
Nothing is left over.
So 3 goes in!
= 21 ÷ 3 = 7 ✓
2 Decide
So 21 is 3 times 7.
It has more factors than 1 and 21.
= 21 = 3 × 7
So 21 is not prime.
= 21 is not prime
Let's look back.
First: try dividing.
Last: decide.
Answer
= 21 is not prime