A man is as fast as a woman and a woman is twice as fast as a boy. If together, they can finish a work in 15 days, how long will a man and a woman take together to finish the same work?

20 days  

                 Man  :  Woman  :  Boy

Efficiency     2    :        2      :      1      

Total efficiency = 2 + 2 + 1 = 5

Total work = efficiency x No. of days = 5 x 16 = 80

Time taken by a boy to complete this same work = \(\frac{Total\; work}{efficiency}\)

                                                                          = \(\frac{80}{4}\) = 20 days