A ladder is supported by a horizontal floor and a vertical wall. Is 15 feet long and weighs 50 lbs, with the weight concentrated at its midlength. The ladder supports a person weighing 180 lbs. The coefficients of static friction between the ladder and the wall, and the ladder and the floor are both 0.30. The angle of the ladder to the floor is 60 degrees. How high can that person climb the ladder before it will slip?