Nairuby code challenge(Divisible Sum Pairs)

nairuby.rb

#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.

def divisibleSumPairs(ar, k)
  ar.combination(2).to_a.select{ |a| (a.index(a[0]) < a.index(a[1])) && ((a[0] + a[1]) % k == 0)}.count
end

array = [1, 2, 3, 4, 5, 6]
x = 5

p divisibleSumPairs(array, x)