#math/mental
In **three digit addition** you process from left to right and sequentially add each place value of the second number in a loop. Consider $342+581$. Read aloud, this becomes
$
\begin{align}
342+\textcolor{#1f9d57}{5}81 & \text{ is} \\
842+\textcolor{#1f9d57}{8}1 & \text{ is} \\
922+\textcolor{#1f9d57}{1} & \text{ is} \\
923&
\end{align}
$