Math operation inside gridview using javascript.

Question

Actually I have to do some calculation inside the Gridview for a stock Management Application.

In the gridview ,

1st column  for Quantity(TextBox)

2nd column for Price/Qty(TextBox)

3rd column for Tax(DropDownlist(which has 3 option-3.5,4.0,0.0))

4th column for Total(Textbox)

 

Suppose Quantity =5

Price/Qty = 10

 Tax=3.5

 Total = (Quantity*Price/Qty)+(Quantity*Price/Qty)*(Tax/100)

 

How ever I was able to do the operation only using textbox-for Tax column

Please help I want to do using a DropDownlist-for Tax column