how to disable text box depends on user choosing text box?

Question

hai  Friends,

  I m creating web application page in that fields are
 
departuredate = textbox
lodging = textbox
food claim = textbox
Daily allowances = textbox


Add(button)


now My requirement is when  the user choose lodging and food claim text boxes daily allowance colmun ll be read only mode. like the same if daily allowances choosed lodging  & food claim read only mode.


how to  make a code?