Need help with probability

[anthonys]

A local public-action group solicits by telephone. For a particular list of prospects, it was estimated that for any individual, the probability was 0.05 of an immediate donation by credit card, 0.25 of no immediate donation but a request for further information through the mail and 0.7 of no expression of interest. Mailed information is sent to all people requesting it, and it is estimated that 20% of these people will eventually donate. An operator makes a sequence of calls, the outcomes of which can be assumed to be independent.

a) What is the probability that no immediate credit card donations will be received until at least four unsuccessful calls have been made?
b) What is the probability that the first call leading up to any donation (either immediate or eventually after a mailing) is preceded by at least four unsuccessful calls?

Ans. a) 0.8145 b) 0.6561

I’m unsure of how to solve this problem and would appreciate some assistance.

[jerryb]

part a is simply the probability of four unsuccessful calls in a row.

so P(u)*P(u)*P(u)*P(u)

i believe that part B is not a well defined problem, so i can't speculate on a solution.

[anthonys]

thank you jerry for the help