diff --git a/2016/ruby/day_15.rb b/2016/ruby/day_15.rb
new file mode 100644
index 0000000..36b9ae0
--- /dev/null
+++ b/2016/ruby/day_15.rb
@@ -0,0 +1,20 @@
+Disc = Struct.new(:positions, :start_position) do
+  def position_at(time)
+    (start_position + time) % positions
+  end
+end
+
+discs = DATA.each_line.map {|line|
+  /Disc #\d has (?<positions>\d+) positions; at time=0, it is at position (?<position>\d+)./ =~ line
+  Disc.new(positions.to_i, position.to_i)
+}
+
+p (0..Float::INFINITY).find {|t|
+  discs.each.with_index.all? {|disc, index|
+    disc.position_at(t + index) == 0
+  }
+}
+
+__END__
+Disc #1 has 5 positions; at time=0, it is at position 4.
+Disc #2 has 2 positions; at time=0, it is at position 1.