diff --git a/2020/ruby/day_05.rb b/2020/ruby/day_05.rb
new file mode 100644
index 0000000..9ec1fb4
--- /dev/null
+++ b/2020/ruby/day_05.rb
@@ -0,0 +1,21 @@
+id = proc {|r, c| r * 8 + c }
+
+seats = ARGF.read.split("\n")
+  .map {|pass| pass.tr("FB", "01").tr("LR", "01") }
+  .map {|pass| pass.to_i(2) }
+  .map {|pass|
+    row = pass >> 3
+    col = pass & 0b111
+    [row, col]
+  }
+# p seats.map(&id).max # day 1
+
+rows = seats.map(&:first).minmax
+cols = seats.map(&:last).minmax
+all_seats = Range.new(*rows).flat_map {|r|
+  Range.new(*cols).map {|c|
+    [r, c]
+  }
+}
+seat = (all_seats - seats).find {|r,_| !rows.include?(r) }
+p id[*seat]