Any Probability Experts?

Hey im doing my homework assignment and can't get a question. If any of you have or are taking a probiblity class and want to try giving this question a shot go for it. Any help would be nice =)

The qestion involves binomial distrobution.

I know A and B so i just need help with C and D
thx

The Canadian Revenue Agency provides phone services for individual income tax enquiries. When the notice of assessment process (meaning the time to get your tax declaration assessed by the Canadian Revenue Agency) is abnormally long, clients may require such phone services to find a solution to their problem. A particular client, who has striven to get his notice of assessment with overwhelming difficulty, assessed that the probability to get a positive and helpful outcome from a phone call is 0.35. This client has an instructor position in a Canadian Business School and teaches business statistics. He is interested in analyzing the required number of phone calls in order to successfully solve an individual income tax problem.
a) Justify why it defines a binomial experiment. What is the random variable?
b) Using the binomial table provided in your textbook (Appendix B, page 583) Determine: P(x=1/n=5), P(x3/n=15)
c) Suppose a tax return problem is fixed after 2 consecutive successful phone calls. Given that the total number of phone calls is 5, what is the probability to solve a client's problem?
d) Suppose this time that on average, 3 successful phone calls are needed (consecutive or not) in order to solve a client's problem. What is the minimum total number of phone calls that guaranties a probability of at least 80% to solve a tax return problem (recommended: start after n=10)?

I know anser to question A and B

A) -There are only 2 outcomes: Helpful/Un-helpful
- The result of one outcome does not influence the result of the next
- The probability of having a helpful phone call stays the same
B) .3125
.2617
.9384