A pharmacist wants to make 30 ml of a 72% solution. How many ml of a 60% solution must he mix with a 80% solution to make the desired solution?

He should use 3/5 80% solution to achieve the desired quantity, or 18 mL of 80% solution and 12 mL of 60% solution. This has a total of 60%*12+80%*18=21.6 mL solution, which is equal to the desired 72%*30=21.6 mL solution.