1 Counting
FreeHow many ways something can happen.
Quant interview prep
Probability for quant interviews
28 short lessons, from counting to continuous probability, on the probability and expected value that trading and quant interviews ask. Every question has a worked solution, and the numbers change each time you practise.
Try one
A classic from trading interviews
Work it out, type your answer, then open the solution to see it done step by step.
Sample question
128 players are put in a random knockout bracket. Every game knocks one player out, and the better player always wins. You pick two of the players at random. What’s the chance they play each other at some point?
Right answers go through as you type.
The course
28 lessons in 8 units
Each unit builds on the one before, and each lesson teaches one method. The first two units are free.
2 Probability basics
FreeThe rules every chance follows.
3 Conditional probability
CoreChances when you already know something.
4 Expected value
CoreAverages, and how much results spread.
5 Distributions
CoreThe patterns behind repeated tries.
6 Betting
CoreWhether a bet is good, and how much to bet.
7 Thinking in states
AdvancedWaits, walks and games that depend on what’s happened so far.
8 Continuous probability
AdvancedRandom numbers in a range, and chances as areas.
How it works
Read it, practise it, drill it
- Step 1
Learn
A concrete problem first, then the method, stated in plain words and in formal notation, with a worked example and the usual mistake to watch for.
- Step 2
Practise
7 questions, the first 2 guided. The rule stays beside the question, and every answer opens a step-by-step solution.
- Step 3
Drill
Each kind of question in the lesson on its own, with new numbers every time, until it’s automatic.
Before the interview
Practise under the same pressure
Mock tests. 10 questions from across the course in 20 minutes, marked at the end and scored out of 100, with a breakdown by topic.
Review. Every kind of question you miss comes back with new numbers, until you’ve got it right on two separate days.
26 classic puzzles. The one-off questions interviewers like, each filed under the lesson whose method solves it.
Puzzles
Questions you’ll recognise
- The Monty Hall problemConditional probability
- Bertrand’s boxBayes’ rule
- The lost boarding passSplitting into cases
- The broken stick triangleProbability as area
- The Pólya urnRandom walks
- An ant on a tetrahedronCollecting every type
- 100 switchesInclusion–exclusion
- Rolling until a repeatComplements
- Two coin waitsGeometric distribution
- The card after the first aceConditional probability
Questions
- What probability do quant interviews ask about?
- Mostly counting, conditional probability and expected value, asked as short puzzles about dice, coins, cards and games. Harder rounds add waiting times, random walks, betting and continuous problems like breaking a stick. The course covers all of these, in that order.
- Do I need calculus?
- Very little. The continuous probability unit works with areas of squares and triangles, and the rest of the course is counting, fractions and short calculations.
- How does a lesson work?
- Each lesson teaches one method. You read it a section at a time, with a few quick questions along the way, then practise on 7 questions: the first 2 walk you through the steps and the other 5 are on your own. Every question comes with a worked solution, and the numbers change each time.
- What’s free?
- The first two units, Counting and Probability basics: 8 lessons with their practice, drills and puzzles, plus one mock test. Every lesson in the course is free to read. Pro opens practice for the other 20 lessons, unlimited mock tests and review, from $0.99 a month.
- Which interviews is this for?
- Trader, quant researcher and quant developer interviews at trading firms, banks and hedge funds, and the probability questions in data science interviews.
Start with counting
Multiplying choices is the first lesson, and the base for everything after it. No account needed to begin.
- Free to start
- Worked solutions
- New numbers every time