A food producer uses two processing plants, P1 and P2, that operate seven days a week. After processing, beef is graded into high-, medium- and low-quality foodstuffs. Highquality beef is sold to butchers, medium-quality beef is used in supermarket readymeals and the low-quality beef is used in dog food. The producer has contracted to provide 120 kg of high-, 80 kg of medium- and 240 kg of low-quality beef each week. It costs $4000 per day to run plant P1 and $3200 per day to run plant P2. Each day P1 processes 60 kg of high-quality beef, 20 kg of medium-quality beef and 40 kg of low-quality beef. The corresponding quantities for P2 are 20 kg, 20 kg and 120 kg, respectively. How many days each week should the plants be operated to fulfil the beef contract most economically?