The factors of 30: 1, 2, 3, 5, 6, 10, 15, 30
The factors of 50: 1, 2, 5, 10, 25, 50
The factors of 100: 1, 2, 4, 5, 10, 20, 25, 50, 100
So their common factors are; 1, 2, 5, 10
and the greatest common factor is: 10
Hope that helps! :D
