I don't know...
Thirty second googling and you have scores of courses on molarity, molality, % concentration, molar fractions and so on, together with tons of questions "How much calcium sulfate is needed to make 100cm3 of 0.25M CaSO4?" or similar. But I was not able to find a single question on the mixing of solutions - and that is not a question that is more basic that molarity definition. In fact when I was a student it was a thing that was taught last...
AWK, are you still teaching 'regula krzyzowa'?
Funny thing, I have tried to search for 'cross-rule' and I have found references to it on Bulgarian and Czech pages only...