You are planning to prepare 600 mL of 20% dextrose solution, by mixing your 5% and 50% dextrose solution. How much of each solution will be needed?

This is an alligation problem, so you will put 50 in the upper left of your tic-tac-toe table, 5 in the lower left, and 20 in the middle. Subtracting across the middle you will put 15 in the upper right and 30 in the lower right. So you need 15 parts of the 50% and 30 parts of the 5%, for a total of 45 parts.
Amount of 50% needed will be:
(15/45) x 600 = 200
Amount of 5% needed will be:
(30/45) x 600 = 400