The seesaw principle: As you probably know, any two persons can balance each other

Question

when sitting on opposite sides of a seesaw, regardless of their weights. It is only necessary
that each person contribute the same moment — calculated by multiplying the person’s
weight times the distance to the fulcrum. For example, if an 80-
pound child sits at one end of a 20-foot plank, 10 feet from the
fulcrum, a 200-pound adult can balance this system. Tell how.

Answer 1

200 x = 10 * 80
x = 4