Compute P(third Ace | Ace in first two)

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Quick Overview

This question evaluates understanding of conditional probability, dependence versus independence, symmetry arguments, and combinatorial reasoning in sampling without replacement.

Compute P(third Ace | Ace in first two)

Company: Citadel

Role: Data Scientist

Category: Statistics & Math

Difficulty: medium

Interview Round: Technical Screen

You draw three cards without replacement from a standard 52-card deck. Given that among the first two cards there is at least one Ace, what is the probability that the third card is an Ace? State and justify any independence or symmetry arguments you use. Then generalize: for a deck of N cards with A Aces, given that the first m cards (m ≥ 2) contain at least one Ace, what is P(card m+1 is an Ace)?

Overview: This question evaluates understanding of conditional probability, dependence versus independence, symmetry arguments, and combinatorial reasoning in sampling without replacement.

Read the full Citadel Data Scientist interview experience this question came from

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Oct 13, 2025
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Conditional probability with cards drawn without replacement

Problem

You draw three cards without replacement from a standard 52‑card deck (4 Aces). Given that among the first two cards there is at least one Ace, what is the probability that the third card is an Ace? State and justify any independence or symmetry arguments you use.

Then generalize: for a deck of N cards with A Aces, given that the first m cards (assume 2 ≤ m < N) contain at least one Ace, what is P(card m+1 is an Ace)?

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