Adiet to contain at least 2400 units of vitamins, 1800 units of minerals, and 1200 calories two foods , food a and food b are to be purchased. each unit of food a provides 50 units of vitamins, 30 units of minerals and 10 calories. each unit of food b provides 20 units of vitamins, 20 units of minerals and 40 calories. if food a costs $2 per unit and food b costs $1 per unit, how many units of each food should be purchased to keep costs at a minimum?