title: Group By, Sum in LINQ
body: <p>suppose I have the following table:</p>
  
  <pre><code>user type amount
  2    a    10
  2    b    20
  2    c    40
  3    a    15
  3    b    20
  3    c    45
  </code></pre>
  
  <p>I want to replace (c) amount with (c - (a+b)) grouping by the user and type, how could I do this? thanks</p>