Prime Numbers
 

A Primes number is a whole number that can only be divided evenly by itself or 1.  Thus, there is only 1 pair of factors ... the number 1 and the number itself.   Examples:   
2 =   1 x 2    Prime
3 =   1 x 3    Prime
4 =   1 x 4, 2 x 2     Not Prime
5 =   1 x 5    Prime
6 =   1 x 6, 2 x 3     Not Prime
7 =   1 x 7    Prime
8 =   1 x 8, 2 x 4     Not Prime
9 =   1 x 9, 3 x 3     Not Prime

 

The  "Sieve of Eratosthenes"  can be used to show prime numbers.   To use the little sieve below:

Click the requests to remove the multiples of 2,3,5,7,and 11 on the left side of the table.   What remains will be the prime numbers less than 100.

The remaining numbers are the prime numbers that are less than 100.

 
 

 

 Remove multiples of 2, except 2.

  2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20

Remove multiples of 3, except 3.

21 22 23 24 25 26 27 28 29 30
31 32 33 34 35 36 37 38 39 40

Remove multiples of 5, except 5.

41 42 43 44 45 46 47 48 49 50
51 52 53 54 55 56 57 58 59 60

Remove multiples of 7, except 7.

61 62 63 64 65 66 67 68 69 70
71 72 73 74 75 76 77 78 79 80

Remove multiples of 11, except 11.

81 82 83 84 85 86 87 88 89 90
91 92 93 94 95 96 97 98 99 100

 

                

Check Your Understanding

  1. Think of a prime number between 2 and 100.
  2. Enter the number below.
  3. Click the button "Check" and check your answer.
Number:

Answer: