title: Find a mistery number in a equation in Ruby
body: <p>So i am learning Ruby and got stuck in one exercise. I need find a mistery number in a equation that is in a string.For example ('10 + ? = 18'). The course i am following ask to use eval. I know eval is dangerous, is just for learning purpose. I am using rspec to test, so dont need use if and return</p>
  <p>All i got is this</p>
  <pre><code>class MisteryNumber
    def calculate(operation)
      count = 0
      expression = operation.split(' = ')
      expression[0].gsub!(&quot;?&quot;, &quot;#{count}&quot;)
      (eval(expression[0] - eval(expression[1])
    end
  end
  </code></pre>
  <p>this code only works if i have subtraction or addition equations. If the equation is like this ('100 / 5 * ? = 40'), doesnt work anymore. I tried to do some form of iterator using gsub(thats why i have the 'count' in the code) but no luck. How i could od this?</p>