In order to avoid sullying the good name of the Wonderbolts by being caught by the authorities, Rainbow Dash's first plan is to anonymously make a large donation of about 1000 tons of Prism Popper candies directly onto Black Bird's house. No, she doesn't mean TO her house. She knows exactly what she said. And by her, she means YOU will anonymously make this rather generous in metric amount by hugely ungenerous in spirit contribution. Oh joy. You're lucky Equestria lacks the means to track deliveries online here.
However, a prank worth doing is worth doing well, and so Rainbow Dash wants to actually send as close to 1000 tons of Prism Poppers to her nemesis as possible. Only problem is, she doesn't actually know how much one individual Prism Popper actually weighs, and while cracking open a pack to check would be simplicity in itself, Rainbow also wants to pack the candies into bags and glue them shut, denying her the sweet treats within, as insult to injury. She happens to have a large pile of packets for you to weigh with Twilight's sensitive electronic scales she stole...borrowed without permission with the intent to return it, but owing to the rather disturbing lack of quality control at the factory (and the fact it's moonlighting as a smuggling organisation on the side), there are some kinks.
Namely, none of the packages Rainbow has contains the same amount of Prism Poppers inside. Aside from that, each packet and all of the delicious contents are identical in every way. Unlikely, but you've been in some pretty unlikely scenarios yourself at this point.
So without being able to actually open a packet to check, how can you determine the weight of a single Prism Popper? For the sake of irony compared to the last time you were allowed a go on the scales, you get unlimited weighings this time.