Exercise 1: Finding the Crossover Point — Possible Solution
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METHOD
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Setting with_adr_cost == without_adr_cost and solving for reversal_cost:

    ADR_COST + wrong_prob * ADR_RISK_REDUCTION * R = wrong_prob * R
    2 + 0.3*0.5*R = 0.3*R
    2 + 0.15*R = 0.3*R
    2 = 0.15*R
    R = 2 / 0.15 = 13.33 hours

RESULTS (verified numerically)
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    reversal_cost=13:    with_adr=3.95, without_adr=3.90 -> no ADR better
    reversal_cost=13.33: with_adr=4.00, without_adr=4.00 -> break-even
    reversal_cost=13.34: with_adr=4.00, without_adr=4.00 -> ADR better
    reversal_cost=20:    with_adr=5.00, without_adr=6.00 -> ADR better

The crossover point is exactly 13.33 hours of reversal cost - below
it, skipping the ADR is the better expected-value choice; above it,
writing the ADR is.

WHY THIS WORKS AS AN ANSWER
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This turns the chapter's own two illustrative examples (1-hour and
200-hour reversal costs) into an actual, general decision rule: any
decision whose reversal would cost more than roughly 13-14 hours of
work is worth the 2-hour ADR investment, given this model's own risk
assumptions. That's a genuinely useful, concrete threshold a team
could actually apply - "would undoing this take more than about two
work days?" - rather than a vague judgment call about whether a
decision "feels important."