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The card sum trading game is a live exercise from trader phone screens and super days at market-making firms (Optiver, IMC, SIG, and Jane Street-style game rounds): the interviewer deals a few cards face down, quotes you a price on their sum, and asks whether you buy or sell. The math is simple — an average card is worth 7, so three cards have an expected sum of 21 — but you get seconds to compare the quote to fair value, pick a side, and state a size.
The part that catches people is the bookkeeping. Rounds keep coming, and at some point the interviewer stops and asks for your running P&L. Quoting the wrong number reads worse than a losing trade, because it says you don't know your own book.
This free version reproduces that pressure: trade a hidden three-card sum contract against a desk price in real time, with variable trade size, a sealed-bid peek auction for an early look at one card (conditional EV), ace-high/low and with/without-replacement toggles, and asymmetric scoring — you submit your own P&L each round, and mis-marking a loss doubles it. Three difficulties cut card visibility from 6 seconds down to 2.5.
An interviewer deals cards face down and quotes a price on their sum. You decide whether to buy or sell relative to expected value, repeat over several rounds, and track your running P&L, which the interviewer will ask you to state. It appears in phone screens and trader rounds at market-making firms.
The average card rank A through K is 7 with ace low (8 with ace high), so n random cards have an expected sum of 7n — three cards are worth 21 on average. If you have seen one card, replace one 7 with its actual value. Buy if the quoted price is below that number, sell if it is above.
Variable trade size for conviction, a sealed-bid peek auction that lets you pay to see one card early and re-price on conditional EV, ace-high/low and with/without-replacement toggles, and know-your-book scoring: you submit your own P&L after each round, and mis-stating a loss doubles it.
Yes. It runs in the browser at quantvault.org with no download. Easy, medium, and hard modes run 5 to 8 rounds, shrinking card visibility from 6 seconds to 2.5 and tightening the answer timer, so the mental-math pressure scales toward real interview pace.