Check a checkbox when textarea is typed in

Check a checkbox when textarea is typed in

I think I have what is a simple question:

I would like a checkbox to be checked if it is not already when a user types in the text area beside it.
Form layout is demo'd below.

  1. <table>
  2.       <tr>
  3.             <td><input type="checkbox" name="box[1]" value = "x"></td>
  4.             <td><textarea name="area[1]"></textarea></td>
  5.       </tr>
  6.       <tr>
  7.             <td><input type="checkbox" name="box[2]" value = "y"></td>
  8.             <td><textarea name="area[2]"></textarea></td>
  9.       </tr>
  10. </table>

Shouldn't be too hard right? but I can't seem to get it to work.
Thanks in advance.