Calculus
posted by chamy .
The seesaw principle: As you probably know, any two persons can balance each other
when sitting on opposite sides of a seesaw, regardless of their weights. It is only necessary
that each person contribute the same moment â€” calculated by multiplying the personâ€™s
weight times the distance to the fulcrum. For example, if an 80
pound child sits at one end of a 20foot plank, 10 feet from the
fulcrum, a 200pound adult can balance this system. Tell how.

200 x = 10 * 80
x = 4