21. The owner of a small motel has an unusual idea to increase revenue. The motel has 20 rooms. He advertises that each night will cost a
base rate of $48 plus $8 times the number of empty rooms that night.
For example, if all rooms are occupied, he will have a total income of
20 * $48-8960 But, if three rooms are empty, then his total income
will be (20-3) ($48 + $8.3) = 17 * $72 = $1224.