add solution to problem 20

problems/20/solution.exs

@@ -0,0 +1,15 @@
+defmodule FactDigitSum do
+  defp fact(1, acc), do: acc
+  defp fact(n, acc), do: fact(n - 1, n * acc)
+  def fact(n), do: fact(n - 1, n)
+
+  defp digit_sum(0, acc), do: acc
+  defp digit_sum(n, acc), do: digit_sum(div(n, 10), rem(n, 10) + acc)
+  def digit_sum(n), do: digit_sum(n, 0)
+end
+
+
+FactDigitSum.fact(100)
+|> FactDigitSum.digit_sum()
+|> IO.puts()
+