You are given an array of n integers, ar = ar[0], ar[1], …, ar[n - 1], and a positive integer, k. Find and print the number of (i, j) pairs where i < j and ar[i] + ar[j] is divisible by k.
For example, ar = [1,2,3,4,5,6] and k = 5. Our three pairs meeting the criteria are [1,4],[2,3] and [4,6].