Joe's Intellectual Newsstand carries a wide variety of magazines . Every Saturday, they order The Economist. The magazine sells an average of 10 copies per day with a standard deviation of 3 copies (normally distributed) for each day from Saturday till the following Friday. Joe buys copies of The Economist for $4 and sells them for $7.50, but when the next edition arrives the following Saturday, he returns any unsold copies and receives $3 trade credit for each returned unit. How many copies of The Economist should Joe's order every week