Owlet puzzle #103
Released Friday, October 16, 2026 UTC
21
the answer for puzzle #103
The five clues
- A two-digit Fibonacci number.
- A two-digit multiple of seven that also appears in the Fibonacci sequence.
- A triangular number (1+2+…+6).
- The product of exactly two primes: 3 × 7.
- In the United States, the traditional legal age for drinking alcohol.
Backstory
21 = F₈ = T₆ = 3 · 7 is one of only four positive integers that is both Fibonacci and triangular (1, 3, 21, 55) — a result proved by Ming Luo in 1989. It's also the number of pips on a standard die and a Blum integer.
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