Today we looked at strategies for calculating prime numbers.

You know that if a number is a multiple of 2, 3, 5 or 7; then it cannot be a prime number. To find this out, a quick way is to use multiples of 10 with known multiplication facts.

For example to discover whether 7 is a factor of 301 then you could use known multiplication facts:

28 is in the 7 times table…

therefore 280 must also be in the 7 times table (as it is 10 times bigger)

now you can count on in 7s…

280, 287, 294, 301!!

Use this strategy to decide whether or not the numbers below the picture are prime.

301

777

257

825

317

46538752

609

825 is not a prime number because you can divide it by 25.

609 is not a prime number because you can divide it by 3 and 6

46538752 is a prime number because you can divide it by 2 4 and 6

317 is a prime number because you can not divide it by any number.

257 is a prime number because you can not divide it by any number.

777 is not a prime number because you can divide it by 7.

301 is not a prime number because you can divide it by 7.

825 is not a prime number because you can divide it by 25.

609 is not a prime number because you can divide it by 3 and 6

46538752 is a prime number because you can divide it by 2 4 and 6

317 is a prime number because you can not divide it by any number.

257 is a prime number because you can not divide it by any number.

777 is not a prime number because you can divide it by 7.

301 is not a prime number because you can divide it by 7.