bonus_drink.rb

class BonusDrink
@@bonusFor = 3

  def self.total_count_for(amount)
    drink = amount #飲める数
    remain = amount #未交換の空き瓶
    while remain >= @@bonusFor
      #商と余りを計算
      changeRes = remain.divmod(@@bonusFor)
    
      #飲める数に交換でもらった分を加算
      drink += changeRes[0]
      
      #未交換分を再計算　 交換でもらった分と余り（未交換）の分
      remain = changeRes[0] + changeRes[1]

    end

    return drink
  end
end