Factors of 18 are 1, 2, 3, 6, 9 , 18 ;
Factors of 72 are 1, 2, 3, 4, 6, 8, 9 , 12 , 18, 24, 36, 72 ;
The factors that are common to all two numbers are 1, 2, 3, 6, 18 ;
The largest of the common factors is 18, so the Greatest Common Factor of 18 and 72 is 18 .