Question

An office block has five floors (ground, 1, 2, 3 and 4), all connected by a lift. When it goes up

to any floor (except 4), the probability that after it has stopped it will continue to rise is 3/4.
When it goes down to any floor (except the ground floor), the probability that after it has
stopped it will continue to go down is 1/4. The lift stops at any floor it passes.
The lift is currently at the first floor having just descended. Calculate the probability of
these events.
a Its second stop is the third floor.
b Its third stop is the fourth floor.
c Its fourth stop is the first floor.

Answer 1

Explanation:

a) The second stop is the third floor:

The first stop is the first floor, so the probability of going up from the first floor is 3/4.

The next stop is the third floor, so the probability of going up from the second floor (which is the first floor in this scenario) to the third floor is 3/4.

The probability of both events happening is the product of the individual probabilities: (3/4) * (3/4) = 9/16.

b) The third stop is the fourth floor:

The first stop is the first floor, so the probability of going up from the first floor is 3/4.

The next stop is the third floor, so the probability of going up from the first floor to the third floor is 3/4.

The next stop is the fourth floor, so the probability of going down from the third floor to the fourth floor is 1/4.

The probability of all three events happening is the product of the individual probabilities: (3/4) * (3/4) * (1/4) = 3/16.

c) The fourth stop is the first floor:

The first stop is the first floor, so the probability of going up from the first floor is 3/4.

The next stop is the third floor, so the probability of going up from the first floor to the third floor is 3/4.

The next stop is the fourth floor, so the probability of going down from the third floor to the fourth floor is 1/4.

The next stop is the first floor, so the probability of going down from the fourth floor to the first floor is 3/4.

The probability of all four events happening is the product of the individual probabilities: (3/4) * (3/4) * (1/4) * (3/4) = 9/64.