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The card sum game comes up in trader phone screens and market-making interviews. The interviewer deals cards face down, quotes a market on their sum and asks what you want to do, and may ask for your running P&L at any point.
This version plays three streets per hand: you trade, a card turns over and the desk quotes again. An optional auction sells a private look at the last card, and the results screen scores your calls, sizing, P&L answers and peek bids out of 100.
An interviewer deals cards face down and quotes a price, or a bid and an ask, on their sum. You buy, sell or pass against the expected value, often over several rounds, and keep track of your P&L. It appears in phone screens and trader rounds at market-making firms.
Count J, Q and K as 11, 12 and 13 and the ace as 14, so the average card is 8 and three hidden cards are worth 24 (with the ace as 1, the average is 7). Once cards turn over from one deck, the hidden cards average what is left, so after a King the fair is about 28.8 rather than 29. Buy when the ask is below your fair, sell when the bid is above it, and pass in between.
Three streets per hand with a new quote after every card, a desk that leans against your trades, and markets where no trade is often the right call. A rival can buy a private look at the last card, so its trades carry information, and every hand ends with a P&L question. There is also a quick 2-hand mode, a daily deal and a review of every decision.
There is a free sample run: four hands at Medium with the peek auction on, scored out of 100, in the browser with no download. Full sessions on every difficulty are part of QuantVault Pro, and harder levels bring tighter, fairer markets and less time per street.