How do I only add/change the variables when I click the Submit button?

I have built a fairly basic Storyline self-assessment. There are 5 pages, with between 5 statements on each page. They rate themselves on each statement from 1-5. I've used a button set for each statement.

They then rate themselves and click Next. At the end of the session, they see their total rating for each Page (a page is a group of statements that go together in a category basically).

My problem is that when they change their mind, ie first rate themselves 1 and then change to 4, that it will add both 1 and 4 to the total score.

How do I change it that it only adds the final button selected; when I click on Submit, those selected must be the only ones added?

I've been adding the variables on the buttons (Example: Add 5 to variable when user clicks the button). I'm not sure how to do it otherwise.