You're so close, but if you look closely 320/3=106.67. However, 320/4=80. And if you look at the other choices, it applies to all of them. 40/4=10 , 80/4=20 , 120/4=30 , and 160/4=40.
You are correct in placing 80 in the box in the table. If you examine the table and the pattern, you'll see that as the number of marbles increases by 10, the weight of the bags increases by 40.
We can determine the rule by dividing 40 by 10: 40 ÷ 10 = 4.
So, the rule is actually to divide the weight of the bag by 4 to find the number of marbles.
Note that this is likely because each marble is 4 grams.
