It takes 4 lawnmowers to cut 2 acres of grass in 2 hours. if the amount of time varies directly with the amount of grass and inversely with the number of lawnmowers, how many hours will it take two lawnmowers to cut 6 acres of grass?

By letting x be the number of lawnmowers, y be the number of acres of grass, and z the number of hours, the joint variation between the variables may be expressed as, 
                                       z = ky / x
where k is the constant of variable. To get k, substitute the values from the first set
                                      2 = k(2) / 4       ; k = 4
To answer the second set,
                                      z = (4)(6) / 2   = 12
Thus, it will take them 12 hours.