Dividing array, question not clear

#1

Could anyone please explain the question to me, and also the expression for difference, it says 1N/2 what does that mean?

#2

lets say at position i and j (i<j) we have the same modulus of prefix array.
now we see that from i+1 pos to jth position , the cumulative mod off array should be zero.
why?
see from 0 to i cumulative mod=k and from 0 to j cumulative mod =k, if we subract these two, cumulative mod will be 0 , i.e. from i+1 to k modulus will be zero. so for all pairs of such i and j , the answer will exist , threfore we take Nc2 of total count to count all such pairs.

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#3

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#4

Got it! Thank you Bhaiya!