Common Factors and Common Multiples

Common Factors 
    The factors of 4 are 1, 2 and 4.
    The factors of 18 are 1, 2, 3, 6, 9 and 18. The numbers 1 and 2 are the factors of both 4 and 18.
    They are the common factors of 4 and 18.
    Two numbers having only 1 as a common factor are called co-prime numbers. Thus, 4 and 15 are 
co-prime numbers.

Common Multiples
    The multiples of 4 are 4, 8, 12, 16, 20, 24, ... 
    The multiples of 6 are 6, 12, 18, 24, 30, 36, ...
   We observe that 12, 24, 36, ... are multiples of both 4 and 6.

They are called the common multiples of 4 and 6.
    Ex.    Find the common multiples of 3, 5 and 6.
    Sol.    Multiples of 3 are 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, ...
        Multiples of 5 are 5, 10, 15, 20, 25, 30, 35, ...
        Multiples of 6 are 6, 12, 18, 24, 30, ...
        Common multiples of 3, 5 and 6 are 30, 60, ...