calculation in textboxes

Hi,
i have four textboxes .adding values in textboxes working fine .Nut if user skips first textbox  and enters value in second textbox  it show displaying that value in another textbox.
 
services        breakfast     lunch               dinner
dropdown    txtbox1 5       txtbox1        txtbox1
dropdown2  txtbox1  5       txtbox1 20         txtbox1
dropdown3  txtbox1         txtbox1         txtbox1
for breakfast if (txt_ServicesBreakfast.Text.Trim() != "") { strBreakfast = Convert.ToString ...