In webform, Can a field be automaticaly filled another field has been fillled by the user

Hi All:
I am new to drupal and trying to do some applications with webform. I want to calculate the value of a field accroding to another.

For example,there are field 1, and field 2.
if user input "5" in the field 1. i want field 2 is filled with "10".(which is 5*2).

I dont know how to do this. Shall I write it in the additional processing? However the additional processing is what be done after the "submit" button is pressed,but i want this value be filled before submit.

Does anyone know any possible solution for this? Really appreciate y'all's help!!

TJ