Emma Jones contracted with the publisher of a GMAT test preparation manual package to sell the review materials at college fairs. She can purchase the book from the wholesaler at $83 per package, with the privilege of returning all unsold packages and receiving a $83 refund per package. She plans to sell the packages for $275 each. She also knows that she must pay $3300 to the organizers of the college fair for the booth rental. She will incur no other costs. If Emma anticipates selling 38 packages, what is her margin of safety in units?