Exercise 2: Why "It's Just Math" Doesn't Make the 7% Guaranteed — Possible Solution
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The math itself is genuinely certain - given a 7% rate, the compound
interest formula reliably produces the same real output every time,
and that part of the calculation is not in question. What's not
certain is whether a real investment will actually earn 7% in any
given year, or on average over 40 real years - the chapter explicitly
calls this figure an assumed rate, based on a commonly cited long-run
average for diversified stock market investing, not a fixed or
promised number.

The friend's reasoning conflates two different things: the
reliability of a mathematical formula once you plug numbers into it,
and the reliability of the real-world assumption feeding into that
formula in the first place. A calculator will always correctly compute
compound growth at 7% - but that doesn't mean any real investment is
guaranteed to actually deliver 7%, since real returns vary year to
year and are never certain in advance, a point the chapter explicitly
flags and the sibling Investing Fundamentals course covers in depth.

ANSWER: This misreads the chapter because it confuses the certainty of
the math itself with the certainty of the 7% assumption feeding into
it. The formula reliably computes what a 7% return would produce -
but 7% is explicitly described as an assumed, illustrative long-run
average, not a guaranteed real-world return, since actual investment
returns vary and are never certain.

WHY THIS WORKS AS AN ANSWER
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This correctly separates mathematical certainty from real-world
uncertainty, directly reproducing the chapter's own explicit warning
about what the example does and doesn't claim, rather than either
agreeing with the friend or dismissing the calculation as unreliable.