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The Entry Record Off by an Hour (illustration)
FILE No.170Unsolved

The Entry Record Off by an Hour

Difficulty ★★☆about 5 minLocked RoomTime offsetsAlibis
The electronic lock's log says the suspect entered at 9:40, but the log's clock was an hour slow.

Case Record

A safe in an office was broken into. The crime happened between 10:30 p.m. and 11 p.m. The electronic lock on the door records the time of entry of the person who used a key.

The suspect's record: entered at 9:40 p.m., with no record of leaving. The suspect claims, "I went in at 9:40 and left before 10. That was before the crime."

However, when it was checked later, the electronic lock's clock was one hour slower than the actual time.

Your question

At what time did the suspect actually enter?

Case Files

Diagram

Log and crime window21:0022:0023:0024:00Crime window 22:30–23:00Log: entry 21:40Suspect: "In at 9:40, left before 10." The lock's clock was one hour slow.
Diagram

Hints

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

Open hint 1

Is the clock fast or slow?

Open hint 2

When it is slow, do you add or subtract to get the actual time?

Open hint 3

Does the corrected time fall within the time of the crime?

Solve It

Your question

At what time did the suspect actually enter?

Choose the answer closest to the truth.

Share: I'm solving this

The Truth

10:40 p.m.

The answer is "10:40 p.m."

The electronic lock's clock is one hour behind the actual time. The log's "9:40" is actually "10:40."

10:40 falls within the time of the crime (10:30 to 11:00). The suspect's claim falls apart. The point is that when a clock is slow you "add." If you subtract, you get 8:40, and the suspect would look innocent instead.

A slow clock is corrected by adding21:0022:0023:0024:00Crime window 22:30–23:00Log 21:40Actual 22:40+1hActual entry was 22:40, inside the crime window.
Diagram of the solution

Key Points

  • Correct the offset of the recording clock first
  • Slow means the actual time is later
  • Do not correct in the wrong direction

Why people get fooled

Because you believe a time shown in a log as it is.

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