I think the number density of skittles or M&Ms in a bag is kind of a weird question to ask. If the volume is just in units of "bags", then it's literally just counting the number of skittles/M&Ms of a certain color. If it is in units of cubic centimeters or the sorts, then there's the difficulty of being able to measure the volume of the bag, which would be able to change once the bag is opened. The only thing I can imagine for this would be dunking the bag in a beaker or large graduated cylinder to measure the volume and then opening the bag to count.
I also think having this in a test would raise certain logistical issues as well. If one bag is provided for all teams, skittles/M&Ms are bound to go missing. If a bag is provided for each team, the exact counts and measurements would vary between teams, making it pretty much impossible to evaluate teams on their accuracy.
Am I not thinking of something here?
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