Jane Street Interview Question: A survey is given to all pass... | Glassdoor

## Interview Question

Assistant Trader Summer Internship Interview(Student Candidate) Princeton, NJ

# A survey is given to all passengers on a number of

different planes. The survey asks each person how full their plane was. The people answer honestly. If 50% of people claim that their plane was 80% full, while the other 50% claim that their plane was 20% full, how full was the average plane?

1

32%, I think? I totally blanked during the interview and it took me a while to get.

Interview Candidate on Feb 17, 2014
7

Let's say each plane holds 100 people. If someone is on a plane which is 80% full, then each of the 80 passengers will say the same thing. Similarly, if someone is on a plane which is 20% full, then each of the 20 passengers say the same thing. We want the number of people who say 80% to be equal to the number of people who say 20%, so for each plane which says 80% there must be four which say 20%. The average is therefore (1/5)80% + (4/5)20% = 32%.

Anonymous on Mar 5, 2014