Given an array of integers and a positive integer k determines the number of (i,j) pairs where i

The solution in python is:

I dont understand why for j in range(n, i+1, -1) why is there i+1 and -1?

and can you please explain the count +=1 ?

def divisiblesumPairs(n,k, ar): count = cor i in range (n): for j in range (n,i+1,1): if (ar [i]+ar[j1])%k==0: count +=1 returncount