sum via stream and enum.ex

defmodule Summer do
  def sum_of_threes_and_fives(n) do
    0..n
    |> Stream.filter(&multiple_of_3_or_5?/1)
    |> Enum.reduce(&Kernel.+/2)
  end

  def multiple_of_3_or_5?(n) do
    rem(n, 5) == 0 || rem(n, 3) == 0
  end
end

defmodule HelloTest do
  use ExUnit.Case

  test "the truth" do
    assert Summer.sum_of_threes_and_fives(3) == 3
    assert Summer.sum_of_threes_and_fives(5) == 8
    assert Summer.sum_of_threes_and_fives(20) == 98
    assert Summer.sum_of_threes_and_fives(999) == 233168
  end
end