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The Clock with Overlapping Hands (illustration)
FILE No.34Unsolved

The Clock with Overlapping Hands

Difficulty ★★★about 6 minTheft CaseClock calculationAlibis
At the moment of the theft the old clock stopped with its hands overlapping. The time is in the 2 a.m. hour. When?

Case Record

A jewel was stolen from the great hall of a mansion. As the thief carried it off, he bumped the old wall clock and it stopped.

The stopped clock had its hour hand (short) and minute hand (long) exactly overlapping. The hands point to the 2 a.m. hour. There is no second hand.

The four suspects’ alibis are as follows.

  • Aoyagi: patrolled the garden from 1:30 to 2:10 a.m. (confirmed by two people)
  • Ishiguro: preparing a late-night snack in the kitchen from 2:08 to 3:00 a.m. (confirmed by the cook)
  • Usami: in a conference call in the study from 12:00 to 2:12 a.m. (on record)
  • Eguchi: in the bath from 2:00 to 2:30 a.m. (confirmed by the housekeeper)
Your question

Who has no alibi at the time in the 2 a.m. hour when the hands overlap?

Case Files

Evidence

Diagram

Early-morning hoursPersonAlibi spanAoyagi1:30-2:10 (patrol)Ishiguro2:08-3:00 (kitchen)Usami0:00-2:12 (conference call)Eguchi2:00-2:30 (bath)The hands overlap and stopped. The time is in the 2 a.m. hour.
Diagram

Timeline

  1. The old clock stops.
  2. The theft of the jewel is discovered.

Hints

If you get stuck, open the hints one at a time, in order.

Open hint 1

How many times in 12 hours do the hour and minute hands overlap?

Open hint 2

At about what time in the 2 o’clock hour do they overlap?

Open hint 3

Who has no alibi at that time?

Solve It

Your question

Who has no alibi at the time in the 2 a.m. hour when the hands overlap?

Choose the answer closest to the truth.

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The Truth

Aoyagi

The answer is Aoyagi.

In 12 hours the hour and minute hands overlap 11 times, at intervals of 12 / 11 hours. The second overlap is at 2 x (12/11) hours, about 2:10:54 (this is the only overlap in the 2 a.m. hour).

At 2:10:54, Ishiguro (2:08 to 3:00), Usami (until 2:12) and Eguchi (2:00 to 2:30) all have alibis. Only Aoyagi finished his patrol at 2:10, so he has no alibi at 2:10:54.

Every 12/11 hoursOverlapAbout1st1:05:272nd2:10:543rd3:16:21
Diagram of the solution

Key Points

  • The hands overlap every 12 / 11 hours
  • The condition "2 a.m. hour" decides the time
  • Check each alibi as a span of time (start to end)

Why people get fooled

If you think roughly that "the hands overlap at about 2:10," the alibis lasting until exactly 2:10 look valid.

Similar tricks

  • The clock-hand calculation type
  • The type where you check the span of an alibi

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