Answer:
42
Step-by-step explanation:
[tex]81-39\\\\\mathrm{Line\:up\:the\:numbers}\\\begin{matrix}\space\space&8&1\\ -&3&9\end{matrix}\\\mathrm{In\:the\:bolded\:column,\:subtract\:the\:second\:digit\:from\:the\:first}\\\begin{matrix}\space\space&8&\textbf{1}\\ -&3&\textbf{9}\end{matrix}\\\\\mathrm{The\:bottom\:number\:is\:larger\:than\:the\:upper\:number.\:\:Try\:to\:'borrow'\:a\:digit\:from\:the\:left.}\\\\\mathrm{Borrow\:}1\mathrm{\:from\:}8\mathrm{.\:\:The\:remainder\:is\:}7\\[/tex][tex]\begin{matrix}\space\space&\textbf{7}&10\\ \space\space&\textbf{\linethrough{8}}&1\\ -&\textbf{3}&9\end{matrix}\\\\\mathrm{Add\:}1\mathrm{\:ten\:to\:}1:\quad \:10+1=11\\\begin{matrix}\space\space&7&\textbf{11}\\ \space\space&\linethrough{8}&\textbf{\linethrough{1}}\\ -&3&\textbf{9}\end{matrix}\\\\\mathrm{In\:the\:bolded\:column,\:subtract\:the\:second\:digit\:from\:the\:first}:\quad \:11-9=2\\[/tex]
[tex]\frac{\begin{matrix}\space\space&7&\textbf{11}\\ \space\space&\linethrough{8}&\textbf{\linethrough{1}}\\ -&3&\textbf{9}\end{matrix}}{\begin{matrix}\space\space&\space\space&\textbf{2}\end{matrix}}\\\mathrm{In\:the\:bolded\:column,\:subtract\:the\:second\:digit\:from\:the\:first}:\quad \:7-3=4\\\\\frac{\begin{matrix}\space\space&\textbf{7}&11\\ \space\space&\textbf{\linethrough{8}}&\linethrough{1}\\ -&\textbf{3}&9\end{matrix}}{\begin{matrix}\space\space&\textbf{4}&2\end{matrix}}[/tex]
