PRACTICE GUIDE JANE STREET
Jane Street Brain Teasers Practice Test
Brain-teaser practice develops logical reasoning, clear explanations and the ability to adapt when a problem changes. Work through the examples, then choose a puzzle from the collection below.
These are Outcry practice puzzles, not Jane Street’s interview questions or a replica of its assessment. Jane Street describes its trading interviews as collaborative problem-solving conversations.
Outcry is not affiliated with Jane Street and has no access to their assessment content. This guide describes an assessment format that candidates report publicly; the questions here are generated by Outcry and are not Jane Street’s own.
What it screens
A quantitative trading firm active across equities, ETFs, bonds, options and currencies, known for OCaml and a strong internship pipeline.
- ✓Identifying assumptions before solving
- ✓Using symmetry and invariants
- ✓Working backward through strategic choices
- ✓Explaining and checking a solution out loud
Where it sits at Jane Street
Use these puzzles to practise reasoning out loud for Jane Street preparation, rather than memorising a trick or final answer. A good explanation states assumptions, checks edge cases and incorporates feedback.
The collection is supplementary skill practice. It does not imply that Jane Street uses a separate brain-teaser test or a particular question count.
The format
These are Outcry's practice-game settings, not verified figures for the firm's assessment.
| Questions | Varies by puzzle |
|---|---|
| Time | Varies by puzzle |
| Negative marking | Varies by firm and year |
| Answer style | Puzzle-specific answers and reasoning |
| Where it sits | Outcry interview skill practice |
What it tests, with a worked example
These worked examples are written by Outcry to explain the skills practised here. They are not the firm's questions.
Check the rules
State what is known and what is only an assumption.
Example
Two boxes are labelled A and B. Exactly one contains a coin. Is box A more likely to contain it?
Answer Not enough information
Knowing exactly one contains a coin does not specify how it was placed. Equal probability requires a symmetric placement rule or another stated assumption.
Use symmetry
Exchangeable positions can eliminate a long calculation.
Example
A fair six-sided die is rolled. Conditional on an odd result, what is the chance it is 5?
Answer 1/3
The remaining outcomes are 1, 3 and 5, all equally likely. One of the three is 5.
Find an invariant
Some properties remain fixed under every allowed move.
Example
Start at 0. Each move adds or subtracts 2. Can you ever reach 7?
Answer No
Every reachable number is even. Adding or subtracting 2 preserves parity, while 7 is odd.
Work backward
Solve the last decision first, then the one before it.
Example
Players alternate removing 1 or 2 stones from a pile of 5. Whoever takes the last stone wins. What should the first player remove?
Answer 2 stones
Leave 3. The opponent must leave either 1 or 2, so you take the remainder. Leaving 4 instead lets the opponent leave you 3.
Count the complement
Calculate the easy opposite event before subtracting from one.
Example
What is the probability of at least one head in three independent fair coin flips?
Answer 7/8
The only excluded outcome is three tails, with probability (1/2)^3 = 1/8. The required probability is 1 - 1/8.
Test a small case
A small counterexample is enough to disprove an unrestricted claim.
Example
A claim says every prime number is odd. Is it correct?
Answer No: 2 is prime and even
A prime is an integer greater than 1 with exactly two positive divisors. Two has divisors 1 and 2, so it is a counterexample. All primes greater than 2 are odd.
What a good score looks like
Aim to explain why your solution works, identify assumptions and respond to a changed rule. No firm pass mark is claimed.
How to train for it
- 01Restate the rules and ask about missing information before calculating.
- 02Try a small case, look for symmetry or a preserved quantity, and explain each step.
- 03Check your answer, then change one assumption and work out whether the reasoning still holds.
TRAIN IT HERE
The drills that match each section
SIT THE FULL BATTERY
All the sections back to back on one clock, marked the way the real screen marks them, with a by-skill breakdown at the end. Included with any pass.
Also reported at Jane Street
Common questions
- Are these real Jane Street questions?
- No. These are independent Outcry practice examples and puzzles.
- How long should I spend on a puzzle?
- Start without a clock so you can explain every step. Then practise giving a clear, concise explanation. Individual games have their own rules and timing.
- What if I get stuck?
- Try the smallest case, write the constraints, look for symmetry or an invariant, and explain what you have ruled out.
- Where can I practise?
- Open Brain Teasers under Train it here and choose a puzzle from the collection.
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