addition_problem_generator.rb

#! /usr/bin/env ruby

class AdditionProblemGenerator

  attr_accessor :max, :number_of_problems

  def initialize(max = 20, number_of_problems = 20)
    self.max = max
    self.number_of_problems = number_of_problems
  end

  def play
    number_of_problems.times do
      first, second = create_problem
      puts "#{first} + #{second} = ?"
      guess = gets
      unless guess.to_i == first + second
        puts "Try again"
        guess = gets
      else
        puts "Great job!"
      end
    end
    puts "That's it! You answered #{number_of_problems} problems!"
  end

  private
  def create_problem
    sum = max + 1
    first = 0
    second = 0

    # Because "adding 0 or 1 is for babies"
    while sum > max || first < 2 || second < 2
      first = rand(max)
      second = rand(max)
      sum = first + second
    end
    [first, second] 
  end
end

if __FILE__ ==  $0
  generator = AdditionProblemGenerator.new(20, 10)
  generator.play
end