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?

20

According to the question, 

1M = 2W = 4B = 5G

Ratio of efficiency  M  :  W  :  B  :  G

                              20  : 10  :  5   :  4

Total work = No. of labour x efficiency x no. of days

            (using the efficiency of a man)

                = 20 x 39 = 780

Efficiency of 1M + 1W + 1B + 1G = 20 + 10 + 5 + 4

                                                 = 39

Time taken = $\frac{Total\; work}{efficiency}$

                  = $\frac{780}{39}$ = 20 days