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What's Left?

Age 5 to 7 Challenge Level:

Alistair from Histon and Impington Infants School says:

When I coloured all of the numbers that could be shared by 2, the even ones were coloured and the odd numbers were left. Then I coloured all the numbers that could be shared by 3. When I tried the numbers that could be shared by 4, there were no new ones because they were even. When I tried the numbers that could be shared by 5, I found they were already coloured.

Alistair sent us a diagram of his results:

numbers 1-20 with 1, 2, 3, 5, 7, 11, 13, 17, and 19 left unshaded

Sim who lives in Singapore explains that the (black) numbers left are 7, 11, 13, 17, and 19, and that they are called prime numbers. Well done.

Sim also points out that 2, 3 and 5 are prime numbers too. I wonder whether anyone can notice something different about 2, the lowest prime number, compared with the others?