Each digit from right to left doubles each time. In the binary system, if the last digit is 1, it's an odd number. 2 is obviously an even number, so the last digit is 0. The second to last digit is 2, the third to last is 4, the fourth to last is 8, and so on. If I want 10 in binary, I need to add 8 and 2 together so that means 10 in binary is 1010. There's already a 2 in binary so we need that 2 only so 10 is 2 in binary.