For example,

15 ÷ 3 = 5		3 and 5 are factors of 15
15 ÷ 6 = 2.5		6 is NOT a factor of 15 because 6 does not divide evenly into 15

Here are some divisibility rules that should help:
Divisibility by 2: If the number ends in 0,2,4,6, or 8, it is divisible by 2.
Divisibility by 3: If the sum of the digits is divisible by 3, the number is divisible by 3.  
   Example: 27 is divisible by 3 because 2 + 7 = 9, which is divisible by 3.
Divisibility by 4: If the last two digits are divisible by 4.
Divisibility by 5: If the number ends in 0 or 5, it is divisible by 5.
Divisibility by 6: If the number is divisible by 2 and 3.
Divisibility by 9: If the sum of the digits is divisible by 9.
Divisibility by 10: If the last digit is 0.

Examples:

Find all the factors of each number in increasing order. Example: The factors of 10 are "1 2 5 10".

12  
25  
28  
31