title: math with instance variables
body: <p>I have this class:</p>
  
  <pre><code>class Account
      attr_accessor :balance
      def initialize(balance)
              @balance = balance
      end
      def credit(amount)
              @balance += amount
      end
      def debit(amount)
              @balance -= amount
      end
  end
  </code></pre>
  
  <p>Then, for example, later in the program:  </p>
  
  <pre><code>bank_account = Account.new(200)
  bank_account.debit(100)
  </code></pre>
  
  <p>If I call the debit method with the "-=" operator in it (as shown in the class above) the program fails with the following message:</p>
  
  <pre><code>bank2.rb:14:in `debit': undefined method `-' for "200":String (NoMethodError)
  from bank2.rb:52:in `&lt;main&gt;'
  </code></pre>
  
  <p>But if I remove the minus sign and just make it @balance = amount, then it works. Obviously I want it to subtract, but I can't figure out why it doesn't work.  Can math not be done with instance variables?</p>