We'll use a compound rule of three to solve your problem:
3 gangs 20km 10 days
x gangs 50 km 15 days
Gangs are directly proportional to the surface and indirectly proportional to the number of days:
[tex]\begin{gathered} \frac{3}{x}=\frac{20}{50}\cdot\frac{15}{10} \\ \frac{3}{x}=\frac{300}{500} \\ \frac{3}{x}=\frac{3}{5} \\ 3x=15 \\ x=5 \end{gathered}[/tex]So, it's needed 5 gangs to complete the work, in total.
