A burger chain is buying both gas and electric grills to make burger patties. A gas grill can cook 20 patties per minute while an electric grill can cook 30 patties per minute. The gas grill requires 20 units of cooking oil per minute while the electric grill requires 25 units of cooking oil per minute. Since the gas grill patties taste better, the number of electric grills must be less than the number of gas grills. Further, the burger chain wants to be able to make at least 150 patties per minute and use at most 140 units of cooking oil per minute. How many of each grill should be bought to minimize the total number of grills in the store?