Question

The Bookworm had 20 copies of Raising Your Rabbit Right!. After a week they had 17 copies left. At the end of two weeks they had 16 copies left and after three weeks they had 13 copies left. At the end of four weeks they had 12 copies left. If they kept selling copies of the book at this same rate, how many copies would they have left at the end of nine weeks?

Answer 1

copies sold per week = (20-12)/4 = 2 copies / week sold
2*9 = 18
20 - 18 = 2

Answer 2

Alternate -3 and -1. At 5 weeks, 9 left, at 6 weeks, 8 left, at 7 weeks, 5 left...and so on.