Newbie question on variables

Hi everybody.

I am not all new to the community but pretty new to Storyline. As I've been working with a couple of authoring tools, most of the things seem pretty clear so far. The one thing I have never been working with are variables. That's good, so I can learn something new.

I want to create a number of exercises helping children to understand the concept of subtraction. An exercise would e.g. be 5-2=3.

To make it easier I want to give a graphic hint. As a prepration they will find a slide with five shapes. Now they are supposed to select the adequate numbers of shapes to substract, for the exercise mentioned that would be two.

Now, somehow I have to count how many shapes they selected.

Those five shapes have the state "Normal" on entering the slide. I add the state "Selected" and a number variable that adds "1" to the value everytime the state of one shape changes to "Selected". That works very well.

And now comes the part that's tricky to me. What happens if one shape to much has been selected, the learner notices and decides to deselect it? I would then have to subtract "1" from my variable. It looks easy, because whenever I click on a selected shape, it's state seems to change to "Normal". But the variable just stays at it was. No subtraction.

I have tried a couple of things e.g. changed the order of the triggers, added another variable for subtracting, but do not arrive at a solution. It is probably just a matter of knowing how for experienced users but me, I am stuck.

To your understanding I add a screenshot of the slide in Storyline.

I would be more than happy if I could get any help on that.

Thanks in advance!

Joe