Question 933473
The description maybe is forcing you to expect percents as mass per volume; otherwise, you may need to know the densities also.  (65% HCl seems very high.)


You can handle the arithmetic in decimal numbers if you want.  You do not really need to turn the percents into decimals.  Here, I WILL.


x for volume of 5%, and y for volume of the "65%".
{{{x+y=42}}} to account for mililiters of volume, and {{{0.05*x+0.65*y}}} is the quantity of PURE HCl.  The concentration as a DECIMAL fraction is {{{(0.05x+0.65y)/42=0.45}}}.   That accounts for the concentration as a decimal fraction.


You know what to do from that?


A strong suggestion is to solve the system for x and y, but do not do any computations for simplification until LAST.