A 5.00 m long uniform ladder leans against a smooth wall and its base rests on a rough floor. The ladder has a mass of 18.0 kg and its base is a distance of 2.30 m from the wall. A person of mass 80.0 kg climbs 2.50 m up the ladder. If the ladder is to remain in place, what frictional force must be exerted by the floor on the ladder?