Challenge 1: A grade Function Using Guards — Possible Solution
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Main.hs:

grade :: Int -> String
grade score
  | score >= 90 = "A"
  | score >= 80 = "B"
  | score >= 70 = "C"
  | otherwise   = "F"

main :: IO ()
main = do
  print (grade 95)
  print (grade 82)
  print (grade 71)
  print (grade 40)

Output:

    "A"
    "B"
    "C"
    "F"

Explanation:
Guards are checked top to bottom, exactly like the chapter's own
classify example -- each `| condition` is tried in order, and the
first one that evaluates to True wins. A score of 82 fails the first
guard (>= 90) but satisfies the second (>= 80), so "B" is returned
without ever checking the remaining guards. otherwise is simply a
guard that's always True, guaranteeing the function has a result for
every possible Int rather than crashing on an unmatched case.

WHY THIS WORKS AS AN ANSWER
------------------------------
This uses guards exactly as the chapter's own classify function
demonstrates, with otherwise as the required fallback, tested against
four different score ranges to confirm each guard fires at the
correct boundary.