given a set of integers, does any non-empty subset of them add up to zero? That is a decision problem and happens to be NP-complete?

No because the subset sum problem is a decision problem in computer science. In its most general formulation, there is a multiset S of integers and a target sum T, and the question is to decide whether any subset of the integers sum to precisely. the problem is known to be NP-complete. moreover, some restricted variants of it are NP-complete too, for example.

1. 0.3 •4 is the same as 4 groups of 0.32. 7•0.8 is the same as 7 groups of 0.83. it increases by 0.5 every time, so 0.5x4 it increases by 0.9 every time, so 0.9x5. 0.9•12=10.86. 0.47•5=2.35i can finish the next ones in a separate post