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    23 · Design problem · hard

    Design a payment system

    The bank debited you, but the app says “Payment failed”. Why?

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    1. 60-second story 9 cards · 1 minOne idea per card. Tap through it like stories.(done)
    2. Quick check 3 questions · 1 minTap an answer, get instant feedback.(done)
    3. Go deeper 12 min5 estimates, 2 build-up diagrams and 6 “you’re the candidate” moments.(done)
    4. Full breakdown 33 min · 14 figuresThe complete breakdown, every number and figure.(done)

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