Chapter $12,$ Problem $39$

Burger Prince buys top$-$grade ground beef for $$1\mathrm{.}00$ per pound. A large sign over the entrance guarantees that the meat $is$ fresh daily. Any leftover meat $is$ sold $to$ the local high school cafeteria for $80$ cents per pound. Four hamburgers can $be$ prepared from each pound $of$ meat. Burgers sell for $60$ cents each. Labor, overhead, meat, buns, and condiments cost $50$ cents per burger. Demand $is$ normally distributed with a mean $of400$ pounds per day and a standard deviation $of50$ pounds per day.

What $is$ the optimal service level for this problem? $(Please$ keep $4$ digits after the decimal point.$)$