I'm not sure if I'm doing this correctly (Chemistry is neither my weakest nor my strongest subject), but from what I understand, if you start with 90 mL water, then you're starting with 90 g water, and if you add 10% NaCl weight for weight, then you get 99 grams, correct? Then using the density, we get (99 grams) / (1.0857 g/mL) = 91.19 mL, which indicates that the volume would increase by 1.3%.
In contrast, it seems that if you use Na2S instead, then the volume would become 88.79 mL instead, which would mean it would decrease.
Is this correct? And so is the answer that "it depends on the solute"?