Elixir function does not return expected result

This is my code which should sum all numbers between 0 and "number" if it is either multiply of 3 or 5, but always return 0

defmodule Challenge do
  def solution(number) do 
    result = 0
    for i <- 0..number-1 do
      if rem(i, 5) == 0 || rem(i, 3) == 0 do
        result = result + i
      end
    end
    result
  end
end

This is test:

defmodule TestSolution do
  use ExUnit.Case
  import Challenge, only: [solution: 1]

  def test_solution(n, expected) do
    assert solution(n) == expected
  end
  
  test "basic tests" do
    test_solution 10, 23
  end
end

In Elixir, variables are immutable, so you can't really change result variable as in your example, it stays within for do code block. Your example would work fine in a OOPS language but Elixir being functional you need to change approach a bit. Elixir provides an Enum module to iterate through a list in much more effective way and your solution can easily be replaced with Enum module.

 defmodule Challenge do
  def solution(number) do 
    0..number-1
    |> Enum.filter(fn x -> rem(x, 5) == 0 || rem(x, 3) == 0 end)
    |> Enum.sum()
  end
end