If 36 mrn can do a piece of work in 25 hours,in how many hours will 15 men do it ?

Explanation:
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.