Answer:
144
Step-by-step explanation:
(81 - 9) * 2 = 144
work backwards, you now have 81 books. But you bought 9, so pretend you didn't buy the 9, means you would have 9 less books. so 81-9.
Then you multiply by 2 because originally you sold half of the books, so before you sold half, you had double the amount, so you multiply that by 2.
