Exercise 2: R-squared for the Sleep/Bugs Model — Possible Solution
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GIVEN
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r ~= -0.985 (from Chapter 7's own sleep/bugs correlation calculation)

STEP 1: APPLYING R-SQUARED = r^2
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Per this chapter's own R^2 = r^2 relationship:

  R^2 = (-0.985)^2 ~= 0.971

STEP 2: INTERPRETING THE RESULT IN PLAIN TERMS
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An R^2 of approximately 0.971 means about 97.1% of the day-to-day
variation in the number of bugs introduced is explained by hours of
sleep alone, in this simple linear model. Only about 2.9% of the
variation is left unaccounted for by the fitted line - a very strong
fit, similar in strength to this chapter's own CPU/response-time
example (R^2 ~= 0.994).

WHY THIS WORKS AS AN ANSWER
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R-squared is computed directly by applying this chapter's own R^2=r^2
shortcut to the already-known r value from Chapter 7, avoiding a
separate recalculation, and the plain-language interpretation states
specifically what percentage of variance is explained versus left
unexplained rather than just reporting the raw number.