diff --git a/21.hs b/21.hs
new file mode 100644
index 0000000..538dde9
--- /dev/null
+++ b/21.hs
@@ -0,0 +1,77 @@
+import Data.List (transpose)
+import Data.List.Split (splitOn, chunksOf)
+import Data.Map.Strict (Map, insert, empty, (!))
+
+type Rules = Map String String
+squareRoot = floor . sqrt . fromIntegral
+
+validTwos   = [
+        [0..3],
+        [0, 2, 1, 3],
+        [1, 0, 3, 2],
+        [1, 3, 0, 2],
+        [2, 0, 3, 1],
+        [2, 3, 0, 1],
+        [3, 1, 2, 0],
+        [3,2..0]
+    ]
+
+validThrees = [
+        [0..8],
+        [0, 3, 6, 1, 4, 7, 2, 5, 8],
+        [2, 1, 0, 5, 4, 3, 8, 7, 6],
+        [2, 5, 8, 1, 4, 7, 0, 3, 6],
+        [6, 3, 0, 7, 4, 1, 8, 5, 2],
+        [6, 7, 8, 3, 4, 5, 0, 1, 2],
+        [8, 5, 2, 7, 4, 1, 6, 3, 0],
+        [8,7..0]
+    ]
+
+valids :: String -> [String]
+valids str = case length str of
+    4 -> map (map (str !!)) validTwos
+    9 -> map (map (str !!)) validThrees
+
+row :: Int -> Int -> String -> [String]
+row multiple size str =
+    map concat . transpose . map (chunksOf multiple) . chunksOf size $ str
+
+derow :: Int -> [String] -> String
+derow multiple sbsq =
+    concat . concat . transpose . map (chunksOf multiple) $ sbsq
+
+chunk :: String -> [[String]]
+chunk str =
+    let size = squareRoot $ length str
+        multiple = if even size then 2 else 3
+        rows = chunksOf (size * multiple) str
+    in  map (row multiple size) rows
+
+dechunk :: [[String]] -> String
+dechunk rows =
+    let multiple = squareRoot . length . head . head $ rows
+    in  concat $ map (derow multiple) rows
+
+enhance :: Rules -> String -> String
+enhance rules grid =
+    dechunk . map (map (rules !)) . chunk $ grid
+
+addRules :: [String] -> String -> Rules -> Rules
+addRules keys value rules = foldr (\key currRules -> insert key value currRules) rules keys
+
+countOn :: String -> Int
+countOn grids = length . filter (== '#') $ grids
+
+parseLine :: String -> Rules -> Rules
+parseLine line rules =
+    let inputStr : outputStr : [] = splitOn " => " line
+        input  = splitOn "/" inputStr
+        output = splitOn "/" outputStr
+    in  addRules (valids $ concat input) (concat output) rules
+
+main :: IO ()
+main = do
+    rules <- foldr parseLine empty . lines <$> readFile "21.txt"
+    let iterations = map countOn $ iterate (enhance rules) ".#...####"
+    print $ iterations !! 5
+    print $ iterations !! 18
\ No newline at end of file
diff --git a/21.txt b/21.txt
new file mode 100644
index 0000000..b16ed3e
--- /dev/null
+++ b/21.txt
@@ -0,0 +1,108 @@
+../.. => .#./.#./###
+#./.. => .#./.#./##.
+##/.. => #../.##/.#.
+.#/#. => #.#/#../..#
+##/#. => ###/##./#.#
+##/## => .../.../.#.
+.../.../... => #.#./.###/..##/#.##
+#../.../... => #.#./#.##/#..#/#..#
+.#./.../... => #.#./###./#.#./..#.
+##./.../... => ##../###./##.#/...#
+#.#/.../... => ..../..#./####/..#.
+###/.../... => ##../..../#.../#...
+.#./#../... => .#../..##/#..#/.#.#
+##./#../... => ####/.###/.###/.###
+..#/#../... => .#.#/.##./...#/##..
+#.#/#../... => #.##/#.##/#.##/.#..
+.##/#../... => .#../...#/..#./.##.
+###/#../... => .##./.###/#..#/##.#
+.../.#./... => .#../.#.#/.#../#.##
+#../.#./... => ...#/##../####/##..
+.#./.#./... => ###./#..#/..#./...#
+##./.#./... => #.##/..#./#.#./..#.
+#.#/.#./... => .#.#/...#/..../#.##
+###/.#./... => ..##/##.#/#.##/###.
+.#./##./... => .##./####/##../####
+##./##./... => .###/..../####/#...
+..#/##./... => ..../##.#/.###/.##.
+#.#/##./... => #.#./###./..../###.
+.##/##./... => ###./.###/.#../##.#
+###/##./... => #.##/#.#./..../##.#
+.../#.#/... => ###./#.##/.###/#.##
+#../#.#/... => ##.#/..../..../.#.#
+.#./#.#/... => .#.#/..##/.#../.##.
+##./#.#/... => .##./..#./...#/#...
+#.#/#.#/... => ..../###./..#./.#.#
+###/#.#/... => ..##/.##./###./#.##
+.../###/... => .#../####/.##./..#.
+#../###/... => ..##/#.#./...#/##..
+.#./###/... => ..#./####/##../#.##
+##./###/... => .##./##.#/####/.#.#
+#.#/###/... => .###/#.##/####/.##.
+###/###/... => #.../#.../##../.##.
+..#/.../#.. => ..##/#.#./#.../#.#.
+#.#/.../#.. => ###./##.#/..#./##.#
+.##/.../#.. => ..#./..../##../.#.#
+###/.../#.. => ####/.#.#/.#.#/####
+.##/#../#.. => ####/####/...#/.#.#
+###/#../#.. => ..##/..#./.##./##..
+..#/.#./#.. => ####/...#/####/#..#
+#.#/.#./#.. => ..#./.###/#.#./##.#
+.##/.#./#.. => .###/.#.#/#..#/..#.
+###/.#./#.. => ..../##../.#.#/.#..
+.##/##./#.. => ###./####/..../#...
+###/##./#.. => ####/#..#/##.#/##.#
+#../..#/#.. => ####/##.#/..../.###
+.#./..#/#.. => ..../.#../..#./..#.
+##./..#/#.. => .#.#/...#/#.##/..#.
+#.#/..#/#.. => #.#./#.##/#..#/####
+.##/..#/#.. => ..#./##../####/.#..
+###/..#/#.. => #.../##.#/###./.#.#
+#../#.#/#.. => ..../.#.#/..#./#.#.
+.#./#.#/#.. => #.##/...#/.##./.#..
+##./#.#/#.. => .###/##.#/##.#/####
+..#/#.#/#.. => ..../..../...#/##.#
+#.#/#.#/#.. => ##.#/.#../###./..#.
+.##/#.#/#.. => ###./..../...#/.##.
+###/#.#/#.. => .###/#..#/.##./.###
+#../.##/#.. => ##../.#.#/.##./.##.
+.#./.##/#.. => .###/.###/..##/.#..
+##./.##/#.. => ..##/###./...#/#...
+#.#/.##/#.. => ..#./###./...#/##..
+.##/.##/#.. => ####/###./#.#./##..
+###/.##/#.. => ..##/.##./#.../..##
+#../###/#.. => ####/.#../.###/.#.#
+.#./###/#.. => .##./##.#/..##/##..
+##./###/#.. => ..##/##.#/##../.#.#
+..#/###/#.. => ##../..../.#.#/#..#
+#.#/###/#.. => ..#./###./####/..##
+.##/###/#.. => ##../##../..##/.##.
+###/###/#.. => ###./...#/#..#/..#.
+.#./#.#/.#. => ..../.###/.###/#...
+##./#.#/.#. => .###/..#./..../#...
+#.#/#.#/.#. => #..#/.##./#.##/..#.
+###/#.#/.#. => ####/##../####/....
+.#./###/.#. => ..../.###/..../###.
+##./###/.#. => ###./.#../#.#./.#..
+#.#/###/.#. => ..../..##/..##/....
+###/###/.#. => ###./...#/#.../..#.
+#.#/..#/##. => ###./.##./.#../....
+###/..#/##. => ####/...#/##../#..#
+.##/#.#/##. => ..../...#/##.#/#.##
+###/#.#/##. => .#.#/.###/..../#...
+#.#/.##/##. => .#.#/#.#./...#/#...
+###/.##/##. => .##./...#/#.../..#.
+.##/###/##. => .#.#/.##./.##./##..
+###/###/##. => #.#./##../##../...#
+#.#/.../#.# => #.#./##.#/##.#/####
+###/.../#.# => .#../.#.#/.##./#.##
+###/#../#.# => ###./##../..##/##..
+#.#/.#./#.# => ####/#.#./###./.##.
+###/.#./#.# => ..#./.##./..../#...
+###/##./#.# => #..#/##.#/.##./.#..
+#.#/#.#/#.# => .#../###./##.#/.#..
+###/#.#/#.# => .#../#.##/##.#/..#.
+#.#/###/#.# => ##.#/.###/..##/.#..
+###/###/#.# => .#../.###/..#./#...
+###/#.#/### => ###./####/.###/#.##
+###/###/### => #..#/.#../#.../...#
diff --git a/README.md b/README.md
index daceb24..1117226 100644
--- a/README.md
+++ b/README.md
@@ -29,9 +29,10 @@ These are the runtimes of only one trial but the variances are fairly small and
 | 18  |  0.118      |
 | 19  |  0.026      |
 | 20  |  0.168      |
+| 21  |  4.013      |
 
 Problems that should be optimized further: 15
 
-Problems that could use some work: 05, 14
+Problems that could use some work: 05, 14, 21
 
 Problems that are good enough with optimizations: 17, 13