jmc

The rules of addition work in base 2 as well as base 10. Carry any sum greater than 1 to the next place value: \begin{array}{r@{}r@{}r@{}r@{}r} & \text{\scriptsize{1}\hspace{0.3mm}} & \text{\scriptsize{1}\hspace{0.3mm}} & \text{\scriptsize{1}\hspace{0.3mm}} & \phantom{1} \\ & 1 & 0 & 1 & 1 \\ + & & 1 & 0 & 1 \\ \hline 1 & 0 & 0 & 0 & 0 \end{array} The sum is $\boxed{10000_2}$.