exp distribution homework??

Here is one of the problems I have, I got part a) already but Im confused about the rest, especially d) and e).

Problem:

1. Partial duration flows (flows greater than some arbitrary base value) have been treated as random variables with a shifted exponential distribution. From 23 years of records during which 56 flow in excess of 5,500 cfs, were observed in a region. The following distribution was found adequate:

http://img229.imageshack.us/img229/3816/distrwx5.jpg

a) Sketch and compute the probability that a flow exceeding 20,000 cfs will be observed, given that the flow exceeds the base value of 5,500. (My answer: 0.1074)

b) What is the probability of at least one flow exceeding 5,500 in one year?

c) Exactly two exceeding 5,500 in one year?

d) Return period of a 20,000 flow?

e) What is the size of the 100 yr flood?