The default mathematical logic would be to multiply them, I would say; it'd need to be defined more clearly to lead me to add them by default.
Uh, no, Marc and Ecth are right. The default mathematical logic is to add and remove 1 to each additional multiplier after the first one.
Indeed, in all cases here where multiplying applies, xN is in fact +(N-1)x100%, as logic dictates since the circumstance for which the multiplier applies individually (meaning there's an addition to the original element quantity).
E.g. +100% = x2
+200% = x3
Multiplying the modifiers would be a mathematical logic error because you'd be adding the original element to itself every time after the first.
E.g. Let's say that CircumstanceA acts as a x2 and CircumstanceB acts as a x2. Both circumstances can apply individually and independantly of each other. Let's call the original element quantity OEQ.
Case1) CircumstanceA can apply: the result is OEQ x 2,
Case2) CircumstanceB can apply: the result is OEQ x 2,
Case3) CircumstanceA and CircumstanceB can apply.
If you multiply the multipliers, you obtain OEQ x 4, meaning you added 300% of OEQ. According to Case1 and Case2, each can only add 100% of OEQ; from where does your additional element quantity come then? The only explanation (that is, aside from it being a mathematical logic error... which it is) would be that Case2 applies to what element quantity Case1 adds but that would cause a problem of logic precedence (is that Case2 that adds an element quantity to what Case1 adds or the reverse?). It only becomes worse as you add circumstances, of course (i.e. multiplying factors).
If you add the multiplier and remove 1 to each additional multiplier after the first one, you obtain OEQ x 3, meaning you added +200% of OEQ, which is coherent since each circumstance affected OEQ, adding +100% of it. Of course, there's also no precedence problem.
Therefore, stacking a x2 on a x3 on a x1.5 on a x4 gives x(2 + (3-1) + (1.5-1) + (4-1)) = x7.5 = +100% + 200% + 50% + 300% = +650%