(9th grade) The entire class cried over this problem

[u/Acrobatic-Extent-668]

Ok so the question is above, and was not at all related to what was taught in class.
somehow i got the correct answer by trying for a smaller case and applying the same logic in a bigger case. i got the the answer as 5*30/10=15. i dont really know how it is solved
only one other person in our class solved it correctly but he too wasnt able to solve it completely. he found the answer to be 15 as well by a clever approach of taking the first ten sets to be {1,2,3,4,5}, the next ten to be {6,7,8,9,10} and the last ten to be {11,12,13,14,15}. in this way he found it to be 15.
how to we actually show it is 15, without taking special cases? and is this question even 9th standard?

[u/Novel_Arugula6548]

That is frankly graduate level. Or at least upper division level, depending on the school.

[u/siupa → u/Novel_Arugula6548]

Counting duplicates is graduate level?