addition and substraction of values for variables based on object states

In storyline 2, I faced some issues when creating an exercise with the following conditions;

1. User have to select 08 components from the list of 13

2. submit button should enable when user selected 08 components

3. If user selects the 09th without submitting, pop up a instruction " deselect any one and select new"

I have developed the exercise, but the issues are;

cant substract the value of "total" variable when user deselects any of selected component.

submit button should enable only on 08 components are selected. If deselect any of selected component change into disable state.

Please any one help me on this regard.

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