If an ever you find yourself working with percentages always recalculate them as fractions. In fact with solutions you are working with probabilities of encountering a given molecule (loosely speaking).
So with mixing solutions: You have X amounts of P(x) solution, and you want to add any amount Y of P(y) solution so it will in the end be a P(z) solution. Now you are adding weighted probabilities.
I taught my then elementary-school-level sister how to convert percentages to fractions and fractions to percentages and she has never since used specialised algorithms for percentages.
If an ever you find yourself working with percentages always recalculate them as fractions. In fact with solutions you are working with probabilities of encountering a given molecule (loosely speaking).
So with mixing solutions: You have X amounts of P(x) solution, and you want to add any amount Y of P(y) solution so it will in the end be a P(z) solution. Now you are adding weighted probabilities.
I taught my then elementary-school-level sister how to convert percentages to fractions and fractions to percentages and she has never since used specialised algorithms for percentages.