dropdown list options display mathematical operations in a text field

Hello,

I have two text fields (x and y) and one dropdown list (w) with three options (1, 2, 3). The text field “x” always has a variable. I want to implement the following idea:

when “1” is selected, the text field “y” to have half (0.5) the value of “x”, i.e. (y=0.5*x),

when “2” is selected, the text field “y” to display 0.25 the value of “x”, i.e. (y=0.25*x),

when “3” is selected, the text field “y” to display “whatever_value” times the value of “x”, i.e. (y=whatever_value*x).

How can I write a javascript code and where to add it?

Example: x = 10, w(1) =>  y = 5;  x = 10, w(2) =>  y = 2.5, ..etc.

I tried the following:

var x= this getField("x").value;

event.value=x*0.5;

but I don't know how to go further.

Thank you.

Silvia