Math problem using Excel

[Trivia]

Say i have Ingredients A and B that form a solution with water. Ingredient A can be added up to 15 % of water solution volume. Ingredient B can be used up to 5 % of solution volume. Ingredients A and B in total can be used up to 15 % of solution volume.

I need a calculator to count how much in excess has Ingredient B been used if any. I've tried it numerous ways but seem to run into the problem that IF has only two possible outcomes so i cant really include all 3 conditions in a way that would take account all three conditions.

 

[Trivia]

So the inputs would have to be Solution volume, amount of ingredient A and amount of ingredient B, outcome should be Excess of ingredient B if there is such.