let x is the distance from the 180 lbs person the fulcrum will be placed
in order to have a balance it should be 180x = 120(15-x) {15-x is the distance from the 120 lbs person}
so it will be; 180x= 1800-120x --->>> 180x+120x = 1800 --->>> 300x = 1800 deviding by 300 both sides
we get; x=6 feet. So the fulcrum must be placed 6 feet from the 180 lbs person in order to have the balance.