32 NUMBERS •^ PRIME NUMBERS
Prime numbers
A prime number is a whole number greater than
1 that can’t be divided by another whole number
except for itself and 1.
A prime number has
only two factors
- itself and 1.
Is 4 a prime number?
We can divide 4 by 1 and by itself. Can
we divide 4 exactly by any other number?
Let’s try dividing by 2: 4 ÷ 2 = 2
We can divide 4 by 2, so 4 is not a prime number.
Is 2 a prime number?
We can divide 2 by 1 and also by itself.
But we can’t divide 2 by any other number.
So, we know 2 is a prime number.
Is 7 a prime number?
We can divide 7 by 1 and by itself. Now let’s try
dividing 7 by other numbers. We can’t divide 7
exactly by 2, 3, or 4. We can stop checking once we
get over half of the number we’re looking at – in this
example, once we get to 4. So, 7 is a prime number.
Is 9 a prime number?
We can divide 9 by 1 and by itself.
We can’t divide 9 exactly by 2, but we
can divide it by 3: 9 ÷ 3 = 3
This means 9 is not a prime number.
Finding prime numbers
To find out whether or not a number is prime, we can try to divide it
exactly by other whole numbers. Let’s try this out on a few numbers.
2 ÷ 1 = 2
2 ÷ 2 = 1
4 ÷ 1 = 2
4 ÷ 4 = 1
9 ÷ 1 = 9
9 ÷ 9 = 1
7 ÷ 1 = 7
7 ÷ 7 = 1
YES
2 is a prime
number
YES
7 is a prime
number
NO
4 is not a
prime number
NO
9 is not a
prime number
4 ÷ 2 = 2
9 ÷ 3 = 3
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