If 36 men can do a piece of work in 25 hours, in how mwny hours will15 men do it?

Let the required no of hours be x. Then

Less men , More hours    (Indirct Proportion)

Therefore,  15:36 ::25:x 

=> (15 x X)=(36 x 25)  => x = 60

Hence, 15 men can do it in 60 hours.