when i click the drop down that value should be come in text box how to do in asp.net with csharp

Question

i am doing application in asp.net(webiste) with csharp code.

design as follows;

                            textbox1     Dropdown1

                            textbox2      dropdown2 ( in drop down 2 0 to 9 values are there  in horzintal)

when i click the dropdown2 that value should come in textbox1 and multiply with dropdown1 and result to be in  textbox2 how to do.