If one man or two women or four boys or five girls can finish a work in 39 days, then how many days will one man, one woman, one boy and one girl together take to finish the same work?

Man × 39 = 2 Women × 39 = 4 Boys × 39 = 5 Girls × 39
\[\begin{array}{*{20}{c}} {}&\begin{gathered} {\text{M}} \hfill \\ \, \downarrow \hfill \\ {\text{20}} \hfill \\ \end{gathered} &\begin{gathered} = \hfill \\ \hfill \\ \hfill \\ \end{gathered} &\begin{gathered} {\text{2W}} \hfill \\ \,\, \downarrow \hfill \\ {\text{10}} \hfill \\ \end{gathered} &\begin{gathered} = \hfill \\ \hfill \\ \hfill \\ \end{gathered} &\begin{gathered} {\text{4B}} \hfill \\ \,\, \downarrow \hfill \\ \,\,5 \hfill \\ \end{gathered} &\begin{gathered} = \hfill \\ \hfill \\ \hfill \\ \end{gathered} &\begin{gathered} {\text{5G}} \hfill \\ \,\, \downarrow \hfill \\ \,\,4 \hfill \\ \end{gathered} \\ {{\text{Efficiency}} \to }&{\text{M}}&{}&{\text{W}}&{}&{\,\,{\text{B}}}&{}&{\,\,{\text{G}}} \end{array}\]
Total work = 20 × 39
20 × 39 = (M + W + B + G) × d
20 × 39 = 39 × d
d = 20 days