Exercise 2: First Guesses from a Frequency Count — Possible Solution

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Using the chapter's English frequency table, E (12.7%) is the most
common letter in ordinary English text by a wide margin, and T (9.1%)
is the second most common. Since Q appears far more often than any
other letter in the ciphertext, and F is the clear second-place
letter, the natural first guess is:

    Q (ciphertext) -> E (plaintext)
    F (ciphertext) -> T (plaintext)

38 out of 300 letters is about 12.7% — a strikingly close match to
E's real-world frequency, which supports the guess further before
any confirmation step is even taken.

What to do next to confirm or refute the guess:

1. LOOK FOR SHORT, COMMON WORDS. If Q really is E, look for a
   3-letter ciphertext group of the pattern X-Y-Q that could plausibly
   be "THE" (with Y as the guessed H and X as the guessed T), since
   "THE" is by far the most common 3-letter English word. If such a
   pattern recurs often in the ciphertext, that's strong supporting
   evidence.

2. CHECK FOR DOUBLED LETTERS. English has common doubled-letter pairs
   (LL, SS, EE, OO). If Q appears doubled somewhere in the ciphertext,
   that's consistent with E occasionally doubling in words like "SEE"
   or "BEEN."

3. CONTINUE DOWN THE FREQUENCY TABLE. Match the third, fourth, and
   fifth most frequent ciphertext letters against A, O, and I/N, and
   see whether tentative decryptions using all these guesses at once
   start producing recognizable English word fragments.

4. BE READY TO REVISE. Frequency-based guesses are a starting
   hypothesis, not a certainty, especially in shorter texts — if
   applying the Q->E, F->T guess produces nonsense fragments, some of
   the initial letter assignments may need to be swapped and
   retested.

WHY THIS WORKS AS AN ANSWER
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This mirrors exactly how the chapter describes frequency analysis
being used in practice: start from the ETAOIN SHRDLU ordering as a
first hypothesis, then refine using short common words and digraph
patterns rather than treating the initial frequency match alone as
proof — the same iterative, evidence-accumulating process the chapter
describes for breaking monoalphabetic substitution generally.