Exercise 3: Ranking Chapter 4's UserManagerBad Against This Chapter's Modules — Possible Solution
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SETUP
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    UserManagerBad (Ch.4): 4 bundled concerns (severity), changed 6 times

    Treating "concerns bundled" the same way this chapter treats
    "copies" - as the count that determines extra effort per change:
      interest = (4 - 1) * 6 = 18

FULL RESULTS (all five items)
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    A (shipping calc):       2 severity, changed 10x -> 10 total interest
    B (notify format):       6 severity, changed 1x  -> 5 total interest
    C (tax lookup):          4 severity, changed 5x  -> 15 total interest
    D (discount rules):      5 severity, changed 8x  -> 32 total interest
    UserManagerBad (Ch.4):   4 severity, changed 6x  -> 18 total interest

    Ranked by total interest paid (highest first):
      D (discount rules): 32
      UserManagerBad (Ch.4): 18
      C (tax lookup): 15
      A (shipping calc): 10
      B (notify format): 5

UserManagerBad lands in second place, above C and below D - a
moderate-severity, moderate-frequency debt item that costs more than
either of the low-frequency, low-to-moderate-severity modules, but
less than the module with the highest severity-frequency combination.

WHY THIS WORKS AS AN ANSWER
------------------------------
This exercise reuses a real prior finding (Chapter 4's own
UserManagerBad, with its 4 bundled concerns discovered via that
chapter's own Exercise 3) as a genuine input to this chapter's
prioritization formula, rather than inventing a new hypothetical
example. It demonstrates the formula isn't limited to duplication
specifically - "severity" is any count of the extra work a change
requires, whether that's duplicate copies (Chapter 8) or bundled
concerns needing separate verification on every touch (Chapter 4) -
and both feed into the same ranking calculation on equal footing.