Chapter $12,$ Problem $39$

Burger Prince buys top$-$grade ground beef for $$1\mathrm{.}00$ per pound. A large sign over the entrance guarantees that the meat is fresh daily. Any leftover meat is sold to the local high school cafeteria for $80$ cents per pound. Four hamburgers can be prepared from each pound of meat. Burgers sell for $60$ cents each. Labor, overhead, meat, buns, and condiments cost $50$ cents per burger. Demand is normally distributed with a mean of $400$ pounds per day and a standard deviation of $50$ pounds per day.$$

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