I said digit, not number. eg. 3 is a number comprised of one digit smaller than 4, and 847 is a number comprised of three digits, each being of their own selves less than 9. 6 (b4) can't exist because 6 is bigger than 4; however 12 (b4) can exist because 1 and 2 are both of their own selves less than 4.
20 (b7) also means there are 2 sevens. 60 (b9) means there are 6 nines. 34 (b5) means there are 3 fives and 4. 17 (b18) means there is 1 eighteen and seven, which means 17 (b18) = 25 (b10).
Also 1 (b2) means there is 1.
EDIT: In equations:
3 = {3} -----> 3 < 4
847 = {8,4,7} ----> 8 < 9, 4 < 9, 7 < 9
6 (b4) cannot exist because 6 > 4
12 (b4) can exist because 12 = {1, 2} -----> 1 < 4, 2 < 4
20 (b7) = 2 x 7
60 (b9) = 6 x 9
34 (b5) = 3 x 5 + 4
thus
17 (b18) = 25 (b10)
1 (b2) = 1