Exercise 1: A Fourth Module in the Ranking — Possible Solution
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THE NEW MODULE
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    D (discount rules): 5 copies, changed 8 times

CALCULATION
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    interest = (copies - 1) * changes = (5 - 1) * 8 = 32

FULL RESULTS (all four modules)
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    A (shipping calc):   2 copies, changed 10x -> 10 total interest
    B (notify format):   6 copies, changed 1x  -> 5 total interest
    C (tax lookup):      4 copies, changed 5x  -> 15 total interest
    D (discount rules):  5 copies, changed 8x  -> 32 total interest

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

Module D becomes the new top priority - not because it's the most
severely duplicated (B still has more raw copies, 6 vs. 5), but
because the combination of moderate-to-high severity and high change
frequency gives it the highest total cost of any module measured so
far.

WHY THIS WORKS AS AN ANSWER
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This directly extends the chapter's own ranking table using the exact
same formula, confirming the formula generalizes to a new module
without any special-casing - and confirming again that the highest
"copy count" (B, 6 copies) is still not the highest-priority debt once
frequency is accounted for.