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Interview Question

Financial Software Developer Intern Interview(Student Candidate) New York, NY

There are 20 floors in a building. If you're on an elevator

  and you're trying to get to the 20th floor, what is the probability that 4 people ahead of you click the 20th floor before you do? Assuming you click last.

Interview Answer

8 Answers


assume there is one button for each floor, so 20 buttons.
a person can press any 1 button out of the 20, prob is 1/20.
Since there are 4 people, so1/16000

ggg on May 30, 2012

These are independent events so the chances of one person before you going to the 20th floor is 1/20. Since this happens 4 times before you the probability is 4*(1/20) or 1/5.

Anna on Jun 3, 2012

The above two are close, but wrong..
There are 20 buttons, thus 20 choices, sure. But you are getting on at one of the floor. No body will press the button for the floor they get on..
Thus, there is really only 19 choices.

P = (1/19)^3 (Independent events mean (1/19)(1/19)(1/19)).

hello on Jun 8, 2012

1/19 + 1/18 + 1/17 + 1/16 assuming that there were no repeated destinations.

Ektor on Aug 8, 2012

based on question:
P(all 4 ahead of you want to get off on 20th fl) = (1/19)^4

real life(all 4 want to get off on 20th fl, and one of them is the first person press the button to 20th fl, and that leave all others, including you, stay still):
(1/19) * (1/4)

Dekus on Sep 17, 2012

about 20% is the right answer. I am surprised with some of the answers, they are all very small possibilities (some less than 1%).

payman k on Oct 2, 2012

I'm quite sure you are all wrong:
The real probability is 1 - P(nobody pushes 20) = 1 - (18/19)^3 = 15%

philly on Feb 16, 2014

1- (19/20)^4

S on Jan 7, 2015

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