Multiples
This is the times tables. Multiples of 5 are 10,15,20,25. Multiples of 7 are 14,28,35,42.
Factors
The factors of a number are all the numbers that go into that number EXACTLY. The factors of 30 are 1,2,3,5,6,10,15 and 30.
Prime Numbers
A prime number can only be divided by 1 and itself. Prime numbers less than 20 are 2,3,5,7,11,13,17 and 19.
A quick trick to work out prime numbers between 21 and 50 is to write out all the number and then cross out all even numbers. Also cross out mulitples of 3,5 and 7. The numbers that are left are all prime numbers.
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DID YOU KNOW:
1 is not a prime number. It only has 1 factor.
2 is the only even prime number.
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