When solving this example: 1001 + 1100 (9 + 12 in decimal form), it would need five single bits to give an answer. The first digits in each number would go to one adder, 1 and 0. It would calculate that 1 + 0 = 1, so that first digit would be 1. The second digits from each are 0 and 0, and 0 + 0 = 0, so the second digit is 0. the third digit from each are 1 and 0, and 1 + 0 = 1, so the third digit is 1. The fourth digit in each are 1 and 1, and 1 + 1 = 10, but because this is a 2 digit number, it holds the first digit (0) and send the second digit to the next adder. The next adder sees 1, and 1 = 1, so the fifth digit is 1. If we take the sum digits in order, they come out as 10101 (21), so 1001 + 1100 = 10101 (9 + 12 = 21).