During a zombie apocalypse, one human finds that about 1 out of every 3 shots he makes

Question:

During a zombie apocalypse, one human finds that about 1 out of every 3 shots he makes actually kills a zombie. Let X be the number of shots he has to take until he kills his fifth zombie.
a. Given that it takes between 14 and 16 shots (inclusive) to kill his fifth zombie, what is the probability that it takes at least 15 shots?
b. Given that it takes him 12 tries to get his 4th success, what is the probability that he will need exactly 3 more tries to get his 5th success? What is this? How do you know?
Distribution
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