$\frac{\begin{matrix}\:\:&1&0&.&0&\bold{1}\\+&0&9&.&9&\bold{0}\end{matrix}}{\begin{matrix}\:\:&\:\:&\:\:&\:\:&\:\:&\bold{1}\end{matrix}}$
Add the digits of the bolded column $0+9=9$$\frac{\begin{matrix}\:\:&1&0&.&\bold{0}&1\\+&0&9&.&\bold{9}&0\end{matrix}}{\begin{matrix}\:\:&\:\:&\:\:&\:\:&\bold{9}&1\end{matrix}}$
Place the decimal point in the answer directly below the decimal points in the terms$\frac{\begin{matrix}\:\:&1&0&\bold{.}&0&1\\+&0&9&\bold{.}&9&0\end{matrix}}{\begin{matrix}\:\:&\:\:&\:\:&\bold{.}&9&1\end{matrix}}$
Add the digits of the bolded column $0+9=9$$\frac{\begin{matrix}\:\:&1&\bold{0}&.&0&1\\+&0&\bold{9}&.&9&0\end{matrix}}{\begin{matrix}\:\:&\:\:&\bold{9}&.&9&1\end{matrix}}$
Add the digits of the bolded column $1+0=1$$\frac{\begin{matrix}\:\:&\bold{1}&0&.&0&1\\+&\bold{0}&9&.&9&0\end{matrix}}{\begin{matrix}\:\:&\bold{1}&9&.&9&1\end{matrix}}$
Write the numbers one under the other, line up the decimal points. $$\begin{matrix}\:\:&1&0&.&0&1\\+&0&9&.&9&0\end{matrix}$ $=19.91$