A physical balance has its arms of length 60 cm and 40 cm. What weight kept on a pan of longer arm will balance an object of weight 100 gf kept on other pan?

As we know, the principle of moments states that

Anticlockwise moment = Clockwise moment

100 gf × 40 cm = W × 60 cm

⇒ W = 100×40\over60 = 66.67 gf

So, a weight of 66.67gf kept on the pan of longer arm will balance the object of weight 100 gf kept on the other pan.