幼女は深さ30メートルの井戸に落ちてしまった。 幼女は1時間ごとに3メートル登るが、その直後に2メートルずり落ちてしまう。 幼女が井戸から脱出するには何時間かかる？

DeepSeek V3.1 (changed mind)

Vote: 28

All models unanimously agree on 28 hours. The key insight is that after 27 hours of net 1m progress (reaching 27m), the 28th hour's 3m climb reaches exactly 30m, allowing escape without slipping. This resolves the common pitfall of assuming 30 hours due to the net 1m rate, ignoring the final climb's lack of slip. The logic is consistent and mathematically sound: 27 hours to reach 27m + 1 hour to climb 3m = 28 total hours.

Sonar Pro (changed mind)

Vote: 28

All five other models have converged on option D (28 hours), and their reasoning is mathematically sound and consistent. The logic is straightforward: the girl makes net progress of 1 meter per hour for the first 27 hours, reaching 27 meters. On the 28th hour, she climbs 3 meters from the 27-meter position, reaching exactly 30 meters and escaping before any slippage occurs. My initial vote of G (31 hours) was incorrect—I appear to have made a computational error. The key insight that all other models correctly identified is that the final 3-meter climb results in escape, so the 2-meter slip-back does not occur. This is a classic well-escape puzzle, and the answer is definitively 28 hours.

GPT-5.4

Vote: 28

私は投票を変えません。答えは28時間です。 ポイントは、毎時間の「登る3m→滑る2m」を最後まで機械的に繰り返すのではなく、30mに到達した瞬間に脱出して滑落が起きないことです。 各1時間の完全なサイクルでは正味1m進むので、27時間後には27m地点にいます。 28時間目の登りで27mから30mに達した時点で井戸の外に出るため、その後の2mの滑りは発生しません。したがって28時間、選択肢Dです。 他モデルの議論も結論は同じで、私の元の判断と一致していました。補強にはなりましたが、立場を変えるほどの新情報はありませんでした。

Claude Opus 4.6

Vote: 28

There is near-unanimous consensus on this answer, and the logic is straightforward and correct. The girl makes a net progress of 1 meter per hour (3m up, then 2m back down). After 27 hours, she is at the 27-meter mark. On the 28th hour, she climbs 3 meters, reaching exactly 30 meters — the top of the well — and escapes. She does not slide back because she has already exited. Therefore, the answer is 28 hours (option D). All models that carefully worked through the calculation arrived at the same answer, including those who initially voted differently but corrected themselves in their reasoning.

Grok 4.1 Fast

Vote: 28

The standard solution holds: net +1m/hour for full cycles (climb 3m, slip 2m). After 27 hours: 27m position. 28th hour: climbs 3m from 27m to 30m, escapes before slipping. Total: 28 hours (D). All models converged on this logic in Round 1, correcting initial errors (e.g., DeepSeek from C, Sonar from G). No compelling arguments for other options; deviations ignore the no-slip on final climb over the top. Alternatives fail: 27h (C) leaves her at 27m pre-final climb; 29h+ (E-H) overcounts unnecessary slips.

Gemini 3.1 Pro

Vote: 28

全モデルの推論を改めて確認しましたが、全員が「1時間あたり実質1メートル登り、27時間経過時点で27メートル地点に到達し、28時間目に3メートル登って脱出する」というロジックで一致しています。 一部のモデル（DeepSeek V3.1やSonar Pro）は初期の選択肢指定でミス（CやG）をしていましたが、推論テキストの中では明確に28時間（選択肢D）が正解であると訂正し、結論づけています。 脱出する最後の1回（28時間目）はずり落ちるプロセスが発生しないという古典的な算数パズルの原則に則り、計算に疑いの余地はありません。したがって、最終ラウンドでも自信を持って「28時間（選択肢D）」に投票します。

Final Summary

The AI Roundtable swiftly converged on a solution to this classic riddle, though DeepSeek V3.1 and Sonar Pro initially stumbled with clerical errors in their option selection. The consensus was anchored by GPT-5.4 and Claude Opus 4.6, who correctly identified that the girl's final 3-meter ascent would place her safely outside the well before she could slip back down.

All 6 models agreed on "28" after discussion

Strongest Arguments

• 28: The girl achieves a net progress of 1 meter per hour for the first 27 hours, reaching 27 meters; on the 28th hour, her 3-meter climb reaches the 30-meter mark exactly, allowing her to escape before the 2-meter slip-back occurs.

Key Turning Points

• DeepSeek V3.1 and Sonar Pro realized their initial choices (C and G) were mathematical or clerical errors after observing the unanimous logic of the other models in the first round.