Jane Street Interview Question: What is the expectation of a ... | Glassdoor.co.uk

Interview Question

Quantitative Trader Interview London, England

What is the expectation of a 6 sided die? What is an

  expectation of a 20 sided die? You have two balls of equal density. One with a diameter of 1.5 ft and the other 1 ft. The 1.5 ft ball weighs 27 lbs. What is the mass of the 1 ft ball?
Answer

Interview Answer

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(1/6)(1+2+3+4+5+6) = 3.5
(1/20)(1+2+...+20) = 10.5 Should've got the trick from the previous question but instead faffed about trying to add 1 to 20 in my head and divide by 20.
27/1.5^3 = m/1^3. m = 8. Realised that 27 was 3^3 far too late and that it is just (3/1.5)^3.

Interview Candidate on 19 Apr 2016

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