The local supermarket buys lettuce each day to ensure really fresh produce. Each morning any lettuce that is left from the previous day is sold to a dealer that resells it to farmers who use it to feed their animals. This week the supermarket can buy fresh lettuce for $4 a box. Each box contains 10 heads of lettuce. The lettuce is sold for $1 a head and the dealer that sells old lettuce is willing to pay $0.20 per head. Past history says that tomorrow's demand for lettuce averages 2500 heads with a standard deviation of 750 heads. How many boxes should the supermarket purchase tomorrow? (A fractional answer is okay).