#math/mental In **two digit subtraction** you process from left to right. When you detect that borrowing is required, round the second number up to the nearest multiple of 10. Subtract. And then add back the difference. Consider $86-29$ $ \begin{align} 86-29 & \text{ equals} \\ 86-30 & \text{ equals} \\ 56 + 1 & \text{ equals} \\ 57 \end{align} $ This extends to **three digit subtraction** by rounding up to the nearest multiple of 100. However, we will be adding back a two-digit difference. This value is quickly detected by seeing the complement of the second number's last two digits. Consider $623-187$. $ \begin{align} 623-1\textcolor{#1f9d57}{87} & \text{ equals} \\ 623-200 & \text{ equals} \\ 423 + \textcolor{#1f9d57}{13} & \text{ equals} \\ 436 \end{align} $