Design real-time fraud scoring
Payments platform. 4,000 transactions per second at peak, 800 average. Every transaction must receive a fraud score before authorisation, and the end-to-end budget for scoring is 50ms at p99.
Fraud labels arrive from chargebacks, 30 to 90 days after the transaction. Roughly 0.15% of transactions are fraudulent. A false negative costs the average transaction value, about £70. A false positive blocks a legitimate customer, and the business estimates the lifetime cost of that at roughly £15 — but it is politically far more visible.
Regulatory requirement: any declined transaction must be explainable to the customer on request, and the decision must be reproducible for two years.
Design this. I want the serving architecture, the feature strategy, the model choice, the threshold decision, and the monitoring — and for each, what you are optimising for and what you are giving up.
Choose how you want to be interrogated
The same case under two modes is two different exercises. If you are unsure, RCA drill is the one that trains root-cause analysis most directly.
RCA drill
A symptom, and nothing else. Find the cause and prove it.
Trains: Hypothesis generation, ranking by prior, and naming the test that discriminates.
Red team
You propose. It attacks. It is trying to make you wrong.
Trains: Defending a position under pressure, and noticing when you should concede.
Socratic
Questions only. It will never tell you the answer.
Trains: Deriving from first principles instead of retrieving from memory.
Design review
Present an architecture. Justify every trade-off you made.
Trains: Stating what you optimised and what you gave up, on purpose.
Viva
Rapid escalating questions. Ends in a graded verdict.
Trains: Answering under time pressure without hedging.
Integration
Problems that span several domains at once. The hardest mode.
Trains: Reasoning across boundaries, where the cause is in a different system to the symptom.
Before you start: you begin locked. The Mentor will give you nothing — not a nudge, not a category — until you state a specific position and the mechanism you think produces it. Four genuine attempts unlock the resolution. There is no shortcut.