diff --git a/app/models/formula.rb b/app/models/formula.rb
index ccd9278..01d9a75 100644
--- a/app/models/formula.rb
+++ b/app/models/formula.rb
@@ -20,7 +20,7 @@ class Formula < ActiveRecord::Base
   def calculate(inputs)
     raise(InvalidInputs, 'No inputs') if inputs.empty?
 
-    deps = inputs.map { |q, v| [q.name, v.transpose[0]] }.to_h
+    deps = inputs.map { |q, v| [q.name, v.transpose.first] }.to_h
     length = deps.values.first.length
 
     raise(InvalidFormula, 'Invalid formula') unless self.valid?
@@ -42,6 +42,13 @@ class Formula < ActiveRecord::Base
 
   private
 
+  class QuantityInput < Array
+    def initialize(q, *args)
+      super(*args)
+      @quantity = q
+    end
+  end
+
   def parse
     d_methods = ['abs', 'nil?']
     q_methods = Hash.new(['all', 'lastBefore'])