A trucker carries cases of Big Red and Dr. Pepper into a region. His truck has the capacity to carry 1000 cases total. He must carry at least 200 cases of Big Red, but no more than 400 cases of Dr. Pepper. Each case of Big Red is charged an import fee of $5 and each case of Dr. Pepper is charged $10. The driver has a budget of $6000 to spend on import fees. If a case of Big Red sells for $20 and a case of Dr. Pepper for $25, how many cases of each should the driver carry so that he maximizes his sales?