mathy.rb

OPERANDS = {zero: 0, one: 1, two: 2, three: 3, four: 4, five: 5, six: 6, seven: 7, eight: 8, nine: 9}
OPERATIONS = {plus: :+, minus: :-, times: :*, divided_by: :/}

def self.method_missing(message, *args)
  if is_target_message? message
    process_message message, *args
  else
    super
  end
end

def process_message message, *args
  call_operand(message, *args) || call_operation(message, *args)
end

def call_operand message, *args
  return false unless is_operand? message
  return OPERANDS[message] if args.empty?
  OPERANDS[message].send args.first[0], args.first[1].to_f
end

def call_operation message, *args
  [OPERATIONS[message], *args]
end

def is_operand? message
  OPERANDS.has_key? message
end

def is_target_message? message
  OPERANDS.merge(OPERATIONS).has_key? message
end

Still a work in progress but not bad, looking pretty clean now and passes all tests.

Not as small as some solutions that were just defining methods on the Object class but I prefer to lean towards a practical solution that would be useable in production. You would never money patch Object like that for a real world application.