diff --git a/2020/ruby/day_21.rb b/2020/ruby/day_21.rb
new file mode 100644
index 0000000..0e82d40
--- /dev/null
+++ b/2020/ruby/day_21.rb
@@ -0,0 +1,26 @@
+require "set"
+
+Food = Struct.new(:id, :ingredients, :allergens)
+
+foods = ARGF.read
+  .scan(/(.+) \(contains (.+)\)/)
+  .map.with_index {|(ingredients, allergens), i|
+    ingredients, allergens = [ ingredients.split(" "), allergens.split(", ") ].map {|x| Set.new(x) }
+    Food.new(i, ingredients, allergens)
+  }
+
+all_ingredients = foods.map(&:ingredients).inject(&:|)
+
+no_allergens = all_ingredients.select {|ingredient|
+  possible_allergens = foods
+    .select {|food| food.ingredients.include?(ingredient) }
+    .map(&:allergens)
+    .inject(&:|)
+  possible_allergens.none? {|allergen|
+    foods_with_allergen = foods.select {|f| f.allergens.include?(allergen) }
+    foods_with_allergen.all? {|f| f.ingredients.include?(ingredient) }
+  }
+}
+
+p foods.map {|f| f.ingredients.count {|i| no_allergens.include?(i) }}.sum
+