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Rooms in a hotel are numbered from 1 to 19.
Rooms are allocated at random as guests arrive.
(a) What is the probability that the first guest to arrive is given a room which is a prime number?
(1 is not a prime number.)

(b) The first guest to arrive is given a room which is a prime number.
What is the probability that the second guest to arrive is given a room which is a prime number?​