The greatest common factor is the greatest number that will divide two values. We have two values 3B and 30B. Each has numbers which multiply together to give the number. We need to find the highest value or most in common they share. Each has the factors:
3B: 1,3,B
30B: 1, 3, 5, 6, 10, B
Both have 3 and B has factors. Our GCF is these two factors.