Owlet puzzle #71
Released Monday, September 14, 2026 UTC
31
the answer for puzzle #71
The five clues
- A two-digit number ending in one.
- A prime one less than a power of two.
- A Mersenne prime: it equals 2^n − 1 for some prime n.
- It equals 2⁵ − 1.
- The reverse of 13, which is also prime — 31 is an emirp.
Backstory
31 = 2⁵ − 1 is the third Mersenne prime, and equals the number of nonempty subsets of a five-element set. In binary it's 11111 — five ones in a row — and it appears as the eleventh prime overall in the sequence 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31.
▶ Play puzzle #71 in practice mode
Or try today's fresh puzzle · Browse the full archive