What is the greatest common factor of 20 and 30?

The greatest common factor of 20 and 30 is 10. This is because 10 is the largest number that divides both 20 and 30 without leaving a remainder. 10 is a factor of both 20 and 30, as it can be obtained by multiplying 2 (a factor of 20) and 5 (a factor of 30). Any number that is a factor of both 20 and 30 must be smaller than or equal to 10, so 10 is the greatest common factor.