Assuming that the probability that the elevator stops at each floor is equal and independent, what is the probability that the elevator stops at least three times from the bottom to the top? What are the most likely stops? The probability that there are 9 points from the ground floor to the tenth floor that can stop once is 9/(9! / 1! +9! /2! +9! /3! +9! /4! +9! /5! +9! /6! +9! /7! +9! /8! +9! /9! )

The probability of stopping twice is 36/(9! / 1! +9! /2! +9! /3! +9! /4! +9! /5! +9! /6! +9! /7! +9! /8! +9! /9! )

The probability of listening to it three times is 84/(9! / 1! +9! /2! +9! /3! +9! /4! +9! /5! +9! /6! +9! /7! +9! /8! +9! /9! )

The probability of stopping four times is 126/((9! / 1! +9! /2! +9! /3! +9! /4! +9! /5! +9! /6! +9! /7! +9! /8! +9! /9! )

. . . . . . . . . .

The probability of nine times is 1/( 1! +2! +3! +4! +5! +6! +7! +8! +9! )

These molecules are C9( 1), C9(2), ... () with the upper corner label and 9 as the lower foot label respectively.

Calculate, not less than three times, and add up the probabilities of four or more times.

If the probability is the highest, the denominator should be changed to the same.