Quant Extern applicants have rated the interview process at Five Rings with 3 out of 5 (where 5 is the highest level of difficulty) and assessed their interview experience as 100% positive. To compare, the company-average is 35% positive. This is according to Glassdoor user ratings.
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There were a number of math questions and then estimation questions and they even allowed me to go back and correct my answer to one of them. The difference is math had to be near exact (I was one off on 3/6 questions and still got the offer) but estimation was a confidence interval.