how do I do this in excel?

15.* The J&B Card Shop sells calendars featuring a different colonial picture for each month. The once-a-year order for each year's calendar arrives in September. From past experience, the September-to-July demand for the calendars can be approximated by a normal distribution with = 300 and standard deviation = 20. The calendars cost $6.50 each, and J&B sells them for $15 each. a. Suppose that J&B throws out all unsold calendars at the end of July. Using marginal economic analysis, how many calendars should be ordered? b. If J&B sells surplus calendars for $1 at the end of July and can sell all of them at this price, how many calendars should be ordered