Can an expression clear a dropdown value if the referring checkbox is unselected?

I'm building a custom form that starts with a checkbox field. Upon selecting an item, it shows a single-select dropdown listing numbers. Then I have a calculation that says, "Sum the value of all those number dropdowns." However, if you deselect an item in the checkbox, it doesn't clear the dropdown value and the calculation is therefore inaccurate. I'm thinking there are two possible workarounds, but I'm struggling to take my thinking further:

1. Write an expression to clear the dropdown value if the checkbox is deselected.
2. In my SUM expression, only sum if the checkbox is also selected.

I'd prefer option 1, but I suspect option 2 is more feasible. What do you think?