What is the best way to go about solving the following question? I can do it the long way but just wanted to know if I could use a formula or something.

Four people are being interviewed for 30 minutes each. Each person is interviewed by Helen for 15 minutes and Jack for 15 minutes. Helen and Jack are in separate rooms and all interviews are between 9:00am and 10:00 am.

a)In how many ways can the interviews be scheduled during the one-hour period?

b) One of the people being interviewed arrived at 9am. What is the probability that he will have his two interviews one after another?

c) Another person being interviewed arrives at 9:00am and hopes to finish in time to leave by 9:50am for another appointment. What is the probability that they will be able to leave on time?