Question 826176
Finish the problem description.  Does he want 10% acid solution, or does he want to mix 10% acid solution with 31% solution?  Does he know how much of each?  Does he know how much acid solution he wants when finished?


What must be clearly mentioned are:
Target concentration,
Concentration of the lesser strength material,
Concentration of the greater strength material,
depending on type of situation - Quantity of expected resulting solution -, 
Quantity of lesser strength material,
Quantity of greater strength material.


Typically, one or two of those would be UNKNOWN variables.