Jane Street's interview "gauntlet" is going viral

Electronic trading firm Jane Street is one of the most notoriously difficult places to get into, not least because of its rigorous hiring process. In the wake of its record-breaking year, various people who failed their Jane Street interviews have gone viral for revealing the questions that stumped them.

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Deedy Das, a partner at VC firm Menlo Ventures, started the trend. In a post that has gathered 8.9m views, Das claimed that Jane Street puts potential interns through a "gauntlet of mental math." He said that he went through six rounds and was stumped by the following question:

"What is the next day after today in DD/MM/YYYY where all the digits are unique?"

The method is just as important as the answer (if not more) when interviewing at Jane Street. Das claimed that the interviewer offered him a pen and paper to do the calculations, but he "knew that was an instant no." After giving an answer, Das was also asked how sure he was of his answer on a scale of 0-1; the interviewer allegedly laughed when he gave an answer of 0.95. Being overconfident can cost you.

Agustin Lebron, a former Jane Street trader, responded to say that he originated the question. He said he "actually stole it from a University Challenge buzz in;" watching the show may now qualify as interview prep.

Following Das' tweet, various people revealed their own tribulations interviewing at Jane Street. Alex Song, a former VP at New York fintech Ramp, claimed that he interviewed there in 2010 while working as a trader for Morgan Stanley. He called it the "worst interview of his life," in which he was given the rules for a card game then told he had an hour to figure out a dominant winning strategy.

Brian Huang, a former trader at rival firm XTX Markets, said he was asked a "maniacal" question by Jane Street while he was at MIT (from 2014 to 2018). The question, responses claimed, is also used for undergraduate math admissions interviews for Cambridge University:

"You are given 30 strings (physical strings, not code strings). 

By tying ends of the strings together and using all of the strings, what is the expected number of loops created? 

For example, you can tie both ends of one string together to form a loop. If you did this with all of the strings, 30 loops would be created. You can also tie two strings together to form a bigger loop. If you did that with all the strings you’d have 15 loops. 

What is the expected number of loops if all of the ends are tied together?"

We can't say for certain whether Jane Street, which did not respond to a request for comment, actually asked these questions. A lot of these anecdotes also took place years, if not decades ago, so the specific questions above could have fallen out of circulation if they are true.

Plenty of responses to Das' tweet came from people who had clearly never interviewed at Jane Street. One joked that they were told to "look for a sign" and that they were later approached in a bar and told to count how many blondes were in the room from memory. Such questions, we can safely assume, will not be asked when you interview for Jane Street. 

Rather than a gauntlet, Jane Street's website says that your "interview will feel a lot more like a conversation than a quiz." It also says that the math required to answer these problems is no more complicated than "what you might learn in a higher-level math course in college."  We've looked at recent interview questions reported online here, and analyze Jane Street's mock interview on how to answer questions here.

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