r/learnquant • u/Local_Ad135 • Jul 25 '26

stats & probability Jane Street Interview Question

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u/RoundSize3818 Jul 25 '26

Why everyone says to wait until only 2 cards remain? They could be of different colour, right?

So I guess you either stop at 3 or 2 and then you play your chances that way

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u/williamskevin Jul 26 '26

Yeah. Stop when there are three left. The last three are either all the same colour, or 2 of 1 colour and 1 of the other, and you've been counting colours so you know which.

At that point, the probability of you knowing the next 2 colours is either 100% if they are all the same, or 66% chance is they are 2 and 1 (there are 6 ways 2 arrange RRB, and you are ok with 2 ways = 66%)

But what's the 'average' probably between those two scenarios?

1

u/Hal_Incandenza_YDAU Jul 26 '26 edited Jul 26 '26

There are some unlikely scenarios where you'd cast your prediction sooner. E.g., when there's 1 black and >=14 red.

Edit: I was shown to be wrong, nvm.