How Traders Train: Market Making Games, Speed, and Math

Trading training targets three separable skills, and they need different practice.

Mental arithmetic

Firms test arithmetic heavily, and candidates often assume it is a filtering gimmick. It is not.

The real reason is attention budget. In a live market you are simultaneously tracking fair value, watching flow, monitoring inventory and deciding on a quote. If computing $\frac{3}{8}$ of 240 consumes your working memory, you have nothing left for the decisions that actually matter. Automatic arithmetic frees attention for judgement.

Worth being fluent in:

• Percentages both ways: 15% of 80, and 12 as a percentage of 80.
• Fractions to decimals, especially eighths, sixteenths and thirty-seconds, which are the tick conventions in several markets.
• Multiplication of two-digit numbers without hesitation.
• Compounding approximations: the rule of 72, and $(1+x)^n \approx 1 + nx$ for small $x$.
• Square roots to two significant figures, since volatility scaling needs them constantly.

The math trainer drills exactly this under a clock, which is the only way it transfers.

Market making games

Simulated quoting is where pricing, inventory and flow reading come together. Beyond the mechanics covered in the previous lesson, the training value comes from repetition with feedback: play, review what drove the P&L, adjust one thing, repeat.

Scenarios firms deliberately construct include sudden price jumps, one-sided flow bursts designed to build inventory, and periods of elevated volatility. Each targets a specific failure: reacting to jumps, managing forced inventory, and sizing spreads to risk.

Reaction speed

Distinct from arithmetic. This is the delay between seeing a change and acting on it, and it is trainable to a point.

More useful than raw reaction time is pre-decision: knowing in advance what you will do in each scenario, so the moment requires recognition rather than deliberation. Experienced traders are not thinking faster, they are thinking less, because the situation is familiar.

Estimation and probability

Firms also test rough quantitative reasoning: Fermi estimation, quick probability calls, and expected value under time pressure. The skill is arriving at a defensible order of magnitude quickly, and being explicit about the assumptions that got you there.

This maps directly onto pricing something with incomplete information, which is the job.

A practice routine that works

Short and frequent beats long and occasional, because these are motor-skill-like abilities that decay without maintenance.

• 10 minutes of mental maths daily.
• A few probability or brainteaser problems, focusing on the ones you get wrong.
• One market making game session, reviewed afterwards.

Traders continue this throughout their careers, not just before interviews. Pricing speed decays without use, and on a desk where decisions are made in seconds, that decay is measurable in P&L.