4. 分数の四則演算（解答）
$$ \begin{eqnarray}
\frac{5}{6} \times \frac{1}{2} – \frac{1}{4}
&=& \frac{5 \times 1}{6 \times 2} &-& \frac{1}{4} \\
&=& \frac{5}{12} &-& \frac{1}{4} \\
&=& \frac{5}{12} &-& \frac{3}{12} \\
&=& \frac{2}{12} \\
&=& \frac{1}{6} \\
\\
\frac{3}{7} \div \frac{9}{5} \div \frac{2}{3}
&=& \frac{3 \times 5 \times 3}{7 \times 9 \times 2}\\
&=& \frac{\color{red}{1} \times 5 \times 3}{7 \times \color{red}{3} \times 2}\\
&=& \frac{1 \times 5 \times \color{blue}{1}}{7 \times \color{blue}{1} \times 2}\\
&=& \frac{5}{14}\\
\\
\frac{5}{9} – \frac{1}{6} \div \frac{9}{8}
&=& \frac{5}{9} – \frac{1 \times 8}{6 \times 9} \\
&=& \frac{5}{9} – \frac{1 \times \color{red}{4}}{\color{red}{3} \times 9} \\
&=& \frac{5}{9} -\frac{4}{27}\\
&=& \frac{15}{27} – \frac{4}{27}\\
&=& \frac{15-4}{27} \\
&=& \frac{11}{27}
\end{eqnarray}
$$