Answer Key 2.8

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

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

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