question

A sandwich salesman wants to determine how many sandwiches to order from the supplier every day. Daily customer demand could be 20 (30% probability), 30 (20% probability), 40 (35% probability) or 50 (15% probability). The salesman buys the sandwiches for $1 and sells them for $2.50. Any sandwiches which are not sold have to be thrown away at the end of that day. What is the optimum level of sandwiches to order? ?20 ?30 ?40 ?50