title: Excel formula to fill a column based on value in another column
body: <p>How can I make a formula such that when update a value in <code>Completed</code> for a particular value of <code>ID</code>, it automatically gets filled in all cells of <code>Completed</code> for that particular <code>ID</code>?  And, when I remove the value from one cell in <code>Completed</code>, it automatically gets removed from all cells in <code>Completed</code> that correspond to that value in <code>ID</code>.</p>
  <p>For eg. in the data below, I'd like the three blank cells automatically filled with <code>4</code>, <code>6</code> and <code>5</code> respectively.</p>
  <pre><code>Role      ID      Completed
   A         1          3
   A         2          4
   A         5          3
   A         8          6
   B         2          
   B         8
   B        10          5
   C        10           
   C        15          2 
  </code></pre>