$20 \ men \rightarrow \frac{c}{2} \ days \\\Rightarrow one \ day \ work \rightarrow \frac{2}{c} \\30 \ men \rightarrow \frac{c}{3}\ days \\\Rightarrow one \ day \ work \rightarrow \frac{3}{c} $
$\\\Rightarrow \ Working \ together \ one \ day \ work \rightarrow\frac{5}{c} \\ Total \ Days \ would \ be \ : \frac{c}{5}$ [