[tex]\bf \begin{array}{ccll}
hours&\stackrel{lawn}{acre}\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
\frac{7}{10}&\frac{2}{5}\\\\
1&a
\end{array}\implies \cfrac{\frac{7}{10}}{1}=\cfrac{\frac{2}{5}}{a}\implies \cfrac{\frac{7}{10}}{\frac{1}{1}}=\cfrac{\frac{2}{5}}{\frac{a}{1}}\implies \cfrac{7}{10}\cdot \cfrac{1}{1}=\cfrac{2}{5}\cdot \cfrac{1}{a}
\\\\\\
\cfrac{7}{10}=\cfrac{2}{5a}\implies 35a=20\implies a=\cfrac{20}{35}\implies a=\cfrac{4}{7}[/tex]
