Originally Posted by

**Ackbeet** I'm afraid you're not thinking about the problem correctly. For each digit, you don't have 10 possibilities, you only have 2. Also, the possibilities don't add, they multiply. Just think about the two-bit case: you've got 00, 01, 10, 11. That's it. 4 possibilities. For 3 bits, you've got 000, 001, 010, 011, 100, 101, 110, 111. That's 8 possibilities. For 4 bits, you have 0000, 0001, 0010, 0011, 0100, 0101, 0110, 0111, 1000, 1001, 1010, 1011, 1100, 1101, 1110, 1111; that's 16 possibilities. Do you see how this is working?