Ruby class method chaining - HELP!

# Ruby version 1.9.3
=begin
Directions: HINT: To do this exercise, you will probably have to use 'return self'.  If the method returns itself (an instance of itself), we can chain methods.

Develop a ruby class called MathDojo that has the following functions: add, subtract. Have these 2 functions take at least 1 parameter. MathDojo.new.add(2).add(2, 5).subtract(3, 2) should perform 0+2+(2+5)-(3+2) and return 4
=end

# define class MathDojo
class MathDojo

  def initialize #constructor
    @sum = 10 #instance variable declared
  end

  def add(*numbers)# adds parameter(s) to instance variable sum
    numbers.inject(@sum) { |sum, number| sum + number }
    self
  end

  def subtract(*numbers) # subtracts parameter(s) from instance variable sum
    numbers.inject(@sum) { |sum, number| sum - number }
    self
  end
end

puts math1 = MathDojo.new.add(2).add(2, 5).subtract(3, 2)