Question 750378
farmer full work complete in 15 days
farmer 1 day work      is =1/15
let his son can complete the work in   x days
son 1 days work   = 1/x
both 1 days work together = 1/15 + 1/x
                          =x+15/15x
as given both can complete task in  6 days
both 1 days task =1/6
equalizing the  value
x+15/15x =1/6
cross multiplication 
6x +90 =15x
9x = 90
x =90/9  =10
therefor son can complete task alone in 10 days

answer  10 days