To find the greatest common factor, first find the largest evenly divisible number that you can take out in both numbers, in this case 15 and 25. Then find the greatest or highest number of each variable that you can evenly take out or divide in both terms, this is for a, b and c.
So GCF of 15 and 25 would be 5
GCF of a = a^1 or a
GCF of b = b^1 or b
GCF of c = c^1 or c
Put everything together to find the GCF.
GCF = 5abc.