The balance in the figure is in equilibrium with balls and bags of sand on each of its plates. The balls are all the same and the bags are also the same. The weight of a bag of sand is equal to the weight of how many balls?

(a) 1

(b) 2

(c) 3

(d) 5

(e) 6

![](https://cdn.mathpix.com/cropped/2024_05_01_666a1aec7e447b0a6cbeg-001.jpg?height=291&width=700&top_left_y=1679&top_left_x=741)
The correct option is (b).

Solution 1: By removing two bags and four balls from each plate, the balance remains level, leaving three bags on the left plate and six balls on the right plate. Therefore, the weight of three bags is equal to the weight of six balls. From this, we conclude that the weight of one bag is equal to the weight of two balls.

Solution 2: Denoting the weight of a bag by $x$ and the weight of a ball by $y$, the balance provides the equation $5 x+4 y=2 x+10 y$, from which it follows that $3 x=5 x-2 x=10 y-4 y=6 y$, that is, $x=2 y$.