Answer Key 2.8

\(\begin{array}{ll}
\begin{array}{cccc}
\text{1st}&&& \\
&&5& \\ \\
&&31& \\ \\
&3&&6
\end{array}
& \hspace{0.5in} \Longrightarrow \hspace{0.5in}
\begin{array}{rrl}
3\cdot 5 + 6&& \\
15+6&=&21
\end{array}
\end{array}\)

\(\begin{array}{ll}
\begin{array}{cccc}
\text{2nd}&&& \\
&&3& \\ \\
&&42& \\ \\
&11&&9
\end{array}
& \hspace{0.5in} \Longrightarrow \hspace{0.5in}
\begin{array}{rrl}
3\cdot 11 + 9&& \\
33+9&=&42
\end{array}
\end{array}\)

\(\begin{array}{ll}
\begin{array}{cccc}
\text{3rd}&&& \\
&&12& \\ \\
&&64& \\ \\
&5&&X
\end{array}
& \hspace{0.5in} \Longrightarrow \hspace{0.5in}
\begin{array}{rrrrl}
5\cdot 12&+&x&=&\phantom{-}64 \\
60&+&x&=&\phantom{-}64 \\
-60&&&&-60 \\
\hline
&&x&=&4
\end{array}
\end{array}\)