Exercise 2: A Bad Sprint Added to the History — Possible Solution
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SETUP
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    PAST_SPRINT_COMPLETIONS = [19, 22, 20, 21, 18, 12]  (added a 6th, bad sprint)

RESULT
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    New known velocity (6 sprints): 18.67 points/sprint
      (down from the chapter's own 5-sprint average of 20.0)

    Sprint committing to exactly the new velocity (18.67):
      completed 18.7, carried over 0, completion rate 100%

The team, committing to its own newly-lowered, honest average,
completes the entire commitment again - the same 100% result the
chapter's own realistic-commitment scenario produced, just at a
slightly lower absolute number.

WHY THIS WORKS AS AN ANSWER
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This confirms the chapter's own core mechanism doesn't depend on
velocity being a specific number - it depends on the COMMITMENT
matching whatever the real, measured velocity actually is, however
that number moves over time. A single bad sprint (illness, an
unexpected outage, a holiday week) genuinely and correctly lowers the
team's own known velocity going forward; the discipline isn't "always
commit to 20" but "always commit to what the last several real sprints
actually showed," recalculated honestly each time, rather than either
ignoring a bad sprint (staying anchored to an outdated, too-high
number) or overcorrecting off a single data point.