Question

how can i find the gratest common factor fast and easy

Answer 1

whats the numbers

Answer 2

Given a and b the number wich you have to find the GCD (Greatst Common divisor, I think that what you are looking for)

Method 1 :
Find the prime decomposition of both number
a = p1 * p2 * p3 ...
b = p'1 * p'2 * p'3 ...
GCD(a,b) = min(p1,p'1) * min(p^2,p'2)

Method 2 :
Euclide's algorithm, lenghtier, but somewhat easier
http://en.wikipedia.org/wiki/Euclidean_algorithm

Answer 3

Find the prime factors of the given numbers and multiply the common factors with no repeats!
Eg., for 6,20
6=2*3
20=2*2*5
Common factors = 2
So, gcd=2