Exercise 1: A PLATINUM Scenario — Possible Solution
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THE SCENARIO
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    def scenario_platinum_member_gets_twenty_percent_off(apply_discount_fn):
        # GIVEN a cart total of $100
        cart_total = 100
        # AND a PLATINUM member checking out
        membership_tier = "PLATINUM"
        # WHEN the discount is applied
        result = apply_discount_fn(cart_total, membership_tier)
        # THEN the total is reduced by 20%
        assert result == 80.0

RESULTS
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    Against original apply_discount: PASS

    Against a changed version where PLATINUM moves from 20% to 25% off:
      FAILED - expected 80.0, got 75.0

WHY THIS WORKS AS AN ANSWER
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This confirms the chapter's own core finding generalizes beyond the
GOLD-tier example it used - any Given-When-Then scenario, for any
policy, fails the moment the policy it describes changes underneath
it, for the exact same reason: the scenario makes a specific,
checkable numeric claim rather than a prose description. The failure
message itself ("expected 80.0, got 75.0") is immediately actionable -
it tells you both what the scenario believed and what the code now
actually does, which is exactly the information a stale static
document could never provide, since a document doesn't know what the
code actually does at all.