Mix 468.75 mL 0.5% solution with 281.25 mL 0.1% solution.
Well, practically, if you are in a lab, the simplest thing to do would be to dilute the 50% solution! But, I assume that they really want you to simply mix the two given solutions together to make the math more interesting.
This is similar to a regular molarity or molality mixing problem. Two volumes of two concentrations equal one volume of another concentration, as long as the dimensions are the same. Two equations in two unknowns – no problem.