We can add the binary numbers 1111_{2} and 110_{2} similar to the way we add the decimal numbers 15_{10} and 6_{10}.

01111 + 00110 1 

1 01111 + 00110 01 

11 01111 + 00110 101 

111 01111 + 00110 0101 

111 01111 + 00110 10101 

111 01111 + 00110 10101 