Citadel Securities Interview Questions

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548 companies
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Last updated Nov 2026

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Quantitative Trader Quantitative Researcher Quantitative Developer

James goes to the gym every Monday and Tuesday, and on any other day with probability 30%. You see him at the gym on a random day. What is the probability it is Monday?
2 years ago
Several random variables all have the same pairwise correlation rho. What values can rho take, for three variables and for n variables?
+110 hours ago
You regress Y on X, then swap them and regress X on Y on the same data. How are the two slopes related, and what happens to R^2?
+110 hours ago
Implement void reverse(char* begin, char* end) in C++ and use it to reverse each word of a string in place, keeping the word order.
2 years ago
Code a single-producer, multiple-consumer ring buffer.
+110 months ago
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Determine whether an element is present in a matrix whose rows and columns are all sorted.
+310 hours ago
Why do you want to work at this firm, and why this role?
+610 hours ago
A drawer holds 4 black and 4 white socks. James takes out four socks. What is the probability that he can form two pairs of same-coloured socks?
2 years ago
Six hamburger patties and five veggie patties are stacked in a random order in a single burger. On average, how many adjacent pairs of patties are of the same type?
2 years ago
X and Y are independent, each uniform on [0,1]. Find the probability that |X - Y| is between 3/5 and 4/5.
2 years ago
20 adults each walk 80 minutes a week on average (SD 10) and 11 children 50 minutes (SD 20), all independently. What is the standard deviation of the total minutes walked?
2 years ago
Yellow buses are late 1/6 of the time and green buses 1/8. A randomly boarded bus is late, and the chance it was green is 1/3. How many yellow buses are there per green bus?
2 years ago
Bags hold 2, 3, 4, 5, 6 candies and each is taken with probability 1/2: what is the probability of taking at least half of all candies?
2 years ago
Each morning for a week Taylor picks eggs or yogurt, independently and with a 50% chance of each. What is the probability of a stretch of at least five consecutive days of eggs?
2 years ago
A is uniform on [1,2], and given A, B is uniform on [1, A^2]. What is the expected value of B?
2 years ago
Five cards numbered 1 to 5 are shuffled. You are told that the first two cards differ by exactly 1. How likely is it that the last card is the 2?
2 years ago
Your coin lands heads 1/6 of the time and your opponent's 1/3 of the time. You alternate flips, starting with you, until one lands heads. What is the probability it is yours?
2 years ago
Four friends each independently and uniformly choose one of 4 restaurants. What is the probability that either all four end up at the same restaurant or all four eat alone?
2 years ago
Two evenly matched teams play games until one has won three. What is the expected difference between the number of games won by the winner and by the loser?
2 years ago
A frog needs two jumps in a row to leave a hole. The first succeeds 20% of the time, the second 30%, and any failure sends it back to the start. Find the expected number of jumps.
2 years ago

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Citadel Securities Interview Questions

James goes to the gym every Monday and Tuesday, and on any other day with probability 30%. You see him at the gym on a random day. What is the probability it is Monday?

Several random variables all have the same pairwise correlation rho. What values can rho take, for three variables and for n variables?

You regress Y on X, then swap them and regress X on Y on the same data. How are the two slopes related, and what happens to R^2?

Implement void reverse(char* begin, char* end) in C++ and use it to reverse each word of a string in place, keeping the word order.

Code a single-producer, multiple-consumer ring buffer.

Determine whether an element is present in a matrix whose rows and columns are all sorted.

Why do you want to work at this firm, and why this role?

A drawer holds 4 black and 4 white socks. James takes out four socks. What is the probability that he can form two pairs of same-coloured socks?

Six hamburger patties and five veggie patties are stacked in a random order in a single burger. On average, how many adjacent pairs of patties are of the same type?

X and Y are independent, each uniform on [0,1]. Find the probability that |X - Y| is between 3/5 and 4/5.

20 adults each walk 80 minutes a week on average (SD 10) and 11 children 50 minutes (SD 20), all independently. What is the standard deviation of the total minutes walked?

Yellow buses are late 1/6 of the time and green buses 1/8. A randomly boarded bus is late, and the chance it was green is 1/3. How many yellow buses are there per green bus?

Bags hold 2, 3, 4, 5, 6 candies and each is taken with probability 1/2: what is the probability of taking at least half of all candies?

Each morning for a week Taylor picks eggs or yogurt, independently and with a 50% chance of each. What is the probability of a stretch of at least five consecutive days of eggs?

A is uniform on [1,2], and given A, B is uniform on [1, A^2]. What is the expected value of B?

Five cards numbered 1 to 5 are shuffled. You are told that the first two cards differ by exactly 1. How likely is it that the last card is the 2?

Your coin lands heads 1/6 of the time and your opponent's 1/3 of the time. You alternate flips, starting with you, until one lands heads. What is the probability it is yours?

Four friends each independently and uniformly choose one of 4 restaurants. What is the probability that either all four end up at the same restaurant or all four eat alone?

Two evenly matched teams play games until one has won three. What is the expected difference between the number of games won by the winner and by the loser?

A frog needs two jumps in a row to leave a hole. The first succeeds 20% of the time, the second 30%, and any failure sends it back to the start. Find the expected number of jumps.

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