http://jsfiddle.net/jamiguel77/ac16w87k/2/

if you write some number on the column of qty2  it react. but i have a error the ids are same (duplicated):

qty3=qty1-qty2

noCajas1

noCajas2

noCajas3

i changed it to:

row1: noCajas1_1

          noCajas1_2

          noCajas1_3

and so on...

in this fiddle:  http://jsfiddle.net/jamiguel77/ac16w87k/3/

this operation: qty3=qty1-qty2    not work anymore, how to fix?

and.................. if user write a char: A   the result is NaN how to prevent it and alert with a errror?

thanks.