Question

An elevator starts at the basement with 8 people (not including the elevator operator) and discharges them all by the time it reaches the top floor, number 6. In how many ways could the operator have perceived the people leaving the elevator if all people look alike to him

Answer 1

1287

Explanation:

Number of people in elevator , n = 8

Number of floors, r = 6

Since they all look alike, arrangement doesn't matter, the we use combination :

(n+r-1 r - 1) = (8+6-1 C 6-1) = 13C5

Recall:

nCr = n! / (n-r)!r!

13C5 = 13! / (13-5)!5!

13C5 = 13*12*11*10*9 / 5*4*3*2*1

= 1287