The Happy Tamal Co. promises its customers delivery within four hours an all tamales orders. All tamales are purchased on the prior day and delivered to the company by 6:00 the next morning, companys daily demand for tamales is as follows:

Number of Tamales Probability

10 0.4

20 0.3

30 0.2

40 0.1

The company purchases tamales for $7 per unit and sells them for $15. All unsold tamales are donated to a local charity society. Store policy is to purchase tamales from a competitor for $15 a unit if a customer orders and the store does not have them. This costs $4 per unit. What is the best quantity to order every single day?