diff --git a/2020/day13/day13.ex b/2020/day13/day13.ex
new file mode 100644
index 0000000..70131d9
--- /dev/null
+++ b/2020/day13/day13.ex
@@ -0,0 +1,133 @@
+defmodule Numbers do
+  use GenServer
+
+  def start_link do
+    primes = [2, 3]
+    max_n = 2
+    GenServer.start_link(__MODULE__, {primes, max_n})
+  end
+
+  def is_prime?(pid, n) do
+    GenServer.call(pid, {:is_prime?, n})
+  end
+
+  def primes_up_to(pid, n) do
+    GenServer.call(pid, {:up_to, n})
+  end
+
+  @impl true
+  def init(primes) do
+    {:ok, primes}
+  end
+
+  @impl true
+  def handle_call({:is_prime?, n}, _from, {primes, max_n}) do
+    IO.inspect({:is_prime?, n, {primes, max_n}})
+    {ans, {primes, max_n}} = check_prime(n, {primes, max_n})
+    {:reply, ans, {primes, max_n}}
+  end
+
+  @impl true
+  def handle_call({:up_to, n}, _from, {primes, max_n}) do
+    {ans, {primes, max_n}} = gen_primes(n, {primes, max_n})
+    {:reply, ans, {primes, max_n}}
+  end
+
+  defp check_prime(n, {primes, max_n}) do
+    if n / 2 <= max_n do
+      {Enum.member?(primes, n), {primes, max_n}}
+    else
+      possible_factors = primes ++ Enum.to_list((max_n + 1)..div(n, 2))
+      has_factor = Enum.any?(possible_factors, &(rem(n, &1) == 0))
+      {!has_factor, {primes, max_n}}
+    end
+  end
+
+  defp gen_primes(n, {primes, max_n}) do
+    if n <= max_n do
+      {Enum.filter(primes, &(&1 <= n)), {primes, max_n}}
+    else
+      new_primes =
+        Enum.reduce((max_n + 1)..n, primes, fn p, primes ->
+          {is_prime, {primes, _}} = check_prime(p, {primes, p - 1})
+
+          if is_prime do
+            primes ++ [p]
+          else
+            primes
+          end
+        end)
+
+      {new_primes, {new_primes, n}}
+    end
+  end
+end
+
+defmodule Day13 do
+  defp read_input do
+    Path.expand('input', Path.dirname(__ENV__.file))
+    |> File.read!()
+  end
+
+  defp test_input do
+    """
+    939
+    7,13,x,x,59,x,31,19
+    """
+  end
+
+  defp parse_int(val, default) do
+    try do
+      String.to_integer(val)
+    rescue
+      ArgumentError -> default
+    end
+  end
+
+  defp parse_schedule(text) do
+    [line_1, line_2] = String.split(text, "\n", trim: true)
+    timestamp = String.to_integer(line_1)
+
+    bus_ids =
+      String.split(line_2, ",")
+      |> Enum.map(&parse_int(&1, nil))
+      |> Enum.filter(& &1)
+
+    {timestamp, bus_ids}
+  end
+
+  # defp lcm(a, b, numbers) do
+  #   bag_union(factors(a, numbers), factors(b, numbers))
+  # end
+
+  # defp factors(n, numbers) do
+  #   Numbers.primes_up_to(numbers, div(n, 2))
+  #   |> Enum.reduce(2..n, {n, %{}}, fn )
+  # end
+
+  # defp bag_union(b1, b2) do
+  #   Enum.reduce(b1, b2, fn {k, v}, bag ->
+  #     Map.put(bag, k, max(v, Map.get(bag, k, 0)))
+  #   end)
+  # end
+
+  defp first_multiple_after(timestamp, bus_id) do
+    if rem(timestamp, bus_id) == 0 do
+      timestamp
+    else
+      bus_id * (div(timestamp, bus_id) + 1)
+    end
+  end
+
+  def part1 do
+    {timestamp, bus_ids} = read_input() |> parse_schedule()
+
+    {bus_id, time} =
+      Enum.map(bus_ids, &{&1, first_multiple_after(timestamp, &1)})
+      |> Enum.min_by(fn {_, time} -> time end)
+
+    bus_id * (time - timestamp)
+  end
+end
+
+Day13.part1() |> IO.inspect()