title: Making 3 cells always sum to 1
body: <p>I have three cells in excel. They must always sum to 1. When the user inputs into any two of the cells, e.g. 0.2 and 0.4, I want the third cell to automatically populate (with 0.4 in this example) so that the three cells together sum to 1.</p>
  
  <p>Hope this is clear.
  Thanks.</p>