A man is twice 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 boy take alone to finish the same work?

105 days 

                 Man  :  Woman  :  Boy

Efficiency      4    :        2     :      1      

Total efficiency = 4 + 2 + 1 = 7

Total work = efficiency x No. of days = 7 x 15 = 105

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

                                                                          = \(\frac{105}{1}\) = 105 days