title: NSDecimalNumber subtraction
body: <p>I need to subtract 0.5 from number a and set the answer to number b. My code looks like it would work but I'm not sure what I'm doing wrong. The error I get Is on the subtraction line, the error says <code>incompatible type for argument 1 of 'decimalNumberBySubtracting:'</code>.</p>
  
  <p>Heres my header: (Note: I only showed the numbers because the header is large)</p>
  
  <pre><code>    NSDecimalNumber *a;
      NSDecimalNumber *b;
  </code></pre>
  
  <p>Heres the rest: (Assume this is in an IBAction)</p>
  
  <pre><code>    b = [a decimalNumberBySubtracting:0.5];
  </code></pre>
  
  <p>If anyone knows how to properly subtract any help would be appreciated.</p>