i have a couple of questions about my basic project

if i have a check box and when its checked, it is going to a text box which is displaying the price, when i uncheck it my price still says in that text box, how can i make it dissapear as i uncheck the box?

```
Dim total As Double
If rb_s1.Checked = True Then
    total += 650.0

txt_1.Text = total

```

thats my code.

and i have many combo boxes, how can i make them all add up as i check/uncheck them.