You need at least two numbers to find something in common.
GCD(24, 90) = 6
GCD: 8
Greatest Common Divisor (GCD) for 18 24 is 6.
The Greatest Common Divisor (GCD) for 17 24 is 1.
Greatest Common Divisor (GCD) for 24 56 72 is 8.
gcd(a,b) = greatest common divisor (or factor, whatever) of a and b. For any set of number, a1, a2, a3, ..., you can use the Euclidean algorithm (http://en.wikipedia.org/wiki/Euclidean_algorithm) to get the gcd(a1, a2), and then again to get gcd(gcd(a1, a2), a3), etc. Often, though, you can eyeball it. So 24*3 = 72. Therefore, gcd(24, 72)=24. 24 does not divide 132, but 12 does (the next biggest factor of 24), so gcd(24, 72, 132)=12.
answer=6
24
4 20 = 22 x 5 24 = 23 x 3 gcd = 22 = 4
56=2*2*2*724=2*2*2*32*2*2=8=GCD
The GCF is 4.
The GCF is 8.