What happens when you have to subtract: 30 -9 Basically 30 - 9 but the 3 must be made 13, so what happens to the zero? I know if it was any other number we would reduce its value by one. Also, if possible, can anyone please show me how 30 in base 5 is 110? I understand that its 1*5*5 + 1*5 + 0*1. But can anyone divide and show me how they arrive to it? I need to do the exercise in the book and I had the same situation as above! Took it to a friend in uni and he couldnt do it either.