Prime Numbers
All numbers can be divided by themselves and by one.
Prime numbers can ONLY be divided by themselves and by one.
Composite numbers can be divided by something else, also.
Example of a
PRIME
 
13 can ONLY be divided by 13 or by 1:

1 group of 13  OR  13 groups of 1



Example of a
COMPOSITE
 
15 can be divided by 5 or 3, as well as by 15 or 1:

3 groups of 5  OR  5 groups of 3



1 group of 15  OR  15 groups of 1



Generally the easiest way to decide whether a number is prime or composite is to run through the list of possible divisors:
Can I divide by 2?
 
(All EVEN NUMBERS can be divided by 2.)

Can I divide by 3?

(Add the digits together if the SUM OF THE DIGITS can be divided by 3, then the number, itself, can be divided by 3.)

Can I divide by 5?

(Does the number end with 0 or 5?)

OTHER PRIMES

Try dividing by 7, 11, 13, 17, 19 and so on.
(HINT: When the answer is less than the divisor you can stop trying.)

Which of these numbers are PRIME? (Click only on the PRIME numbers.)
[If you don't have a Javaenabled browser, there is an answer key at the bottom of the page.]
12
17
21
25
27
30
37
39
REMEMBER: Prime numbers can ONLY be divided by themselves and by one.
Composite numbers can be divided by something else, also.
1) Which of these is a prime number?
2) Which of these is a prime number?
3) Which of these is NOT a prime number?
4) Which of these is a prime number?
5) Which of these is NOT a prime number?
ANSWERS:
Primes: 17 and 37;
the others are composite numbers:
12 = 2 x 2 x 3 = 4 x 3 = 2 x 6
21 = 3 x 7
25 = 5 x 5
27 = 3 x 3 x 3 = 3 x 9
30 = 2 x 3 x 5 = 2 x 15 = 3 x 10 = 6 x 5
39 = 3 x 13
1) E 2) C 3) D 4) A 5) D

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