very simple question here for a Guru
given 2 text box how can I change the value of 1 text box by entering a value in another text box and vice versasome example
  change the 0 for a 2 and you get   //20 and 0 are values from db
change  for  and you get  
change  for  and you get   here by going over 20 I do not want to get a negative number in the other box.
What if I have multiple boxes dynamically generated (multiple pair).
This is in relation with another question I have open where I may be asking to much so if I can have the basic I may be able to work something out.