Math Problem Solver

$$21 - 12=9$$

$$\begin{aligned} \overset{\mathtt{{\scriptscriptstyle 1}}}{\cancel{\mathtt{2}}}\overset{\mathtt{{\scriptscriptstyle 11}}}{\cancel{\mathtt{1}}}\\ \mathtt{-\phantom{0}}\mathtt{1}\mathtt{2}\\ \hline \mathtt{0}\mathtt{9} \end{aligned}$$

Borrow $10^{1}$, resulting in $1$ in the $10^{1}$ place, and $11$ in the $10^{0}$ place.

$9$ is the digit in the $10^{0}$ place. $11 \times 10^{0} - 2 \times 10^{0} = 9 \times 10^{0}$.

$0$ is the digit in the $10^{1}$ place. $1 \times 10^{1} - 1 \times 10^{1} = 0 \times 10^{1}$.