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Selling Calendars:

An Example Problem with Probability

Your store is selling calendars, which cost you $6.00 and sell for $11.99.

You cannot predict demand for the calendars with certainty. Data from
previous years suggest that demand is well described by a "Poisson"
random variable with mean value 70 (Note: we'll discuss exactly what that means
in increasing detail over the next few weeks.)

Calendars which remain unsold after January are returned to the
publisher for a $3.00 "salvage" credit.

There is only one opportunity to order the calendars. What is the right
number of calendars to order?