Exercise 2: A Second Lifespan Comparison — Possible Solution
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SETUP
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    severity = 5 copies -> extra_edits_per_change = 5 - 1 = 4

    Feature flag, deleted after 3 weeks, changed once before deletion:
      interest = 4 * 1 = 4

    Same severity, kept permanent, changed 20 times:
      interest = 4 * 20 = 80

RESULTS
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    Feature flag (deleted young):     4 total interest
    Same code kept permanent:        80 total interest
    Ratio: 80 / 4 = 20.0x

Identical debt severity, twenty times the cost, purely as a result of
how long the code stayed in the codebase changing.

WHY THIS WORKS AS AN ANSWER
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This reproduces the chapter's own lifespan finding (there: a 25x ratio
at severity 3, changes 2 vs. 50) at a different severity and change
count, confirming the underlying relationship - interest scales
directly with how long a piece of debt-carrying code keeps
accumulating changes - holds generally, not just for the chapter's own
specific numbers. The exact ratio differs (20x here vs. 25x in the
chapter) because the change counts differ, but the qualitative
conclusion is identical: code confirmed to be short-lived pays
dramatically less interest for the same severity of debt.