For more practice, try Problem 2.9, parts (c) and (d). (This is optional -- you do not have to hand in Problem 2.9.)
This classic mathematical problem goes back to Leonardo Fibonacci (? - ca 1250). To get a neat formulation, we're going to make the extreme assumptions that every pair of rabbits matures in one month, and produces a pair of baby rabbits the month after reaching maturity and every month thereafter. Start with one pair of baby rabbits at the beginning of Month 0. At the beginning of Month 1 this pair matures, but there will still be only one pair of rabbits. By the beginning of Month 2, however, there will be two pairs: the original pair, plus one new baby pair born to that original pair. By the beginning of Month 3, there will be only one more pair, for a total of three pairs, because the baby pair is not yet able to reproduce. By the beginning of Month 4, however, there will be a total of five pairs, three from the preceding month, plus two more born to the pairs that were mature that preceding month.
(This problem is from K. Steiglitz, A Digital Signal Processing Primer, Addison-Wesley, 1996, page 195.)