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Who Broke the Promise (illustration)
FILE No.160Unsolved

Who Broke the Promise

Difficulty ★★☆about 5 minEveryday MysteryRoutesSame person
Nobody broke a promise, yet it came out that he broke the vase. Why?

Case Record

At an elementary school with only six students, the teacher Ms. K's treasured vase was broken. The one who broke it was Boy A. On the way home, Boy A consulted four others, Boy B, Boy C, Girl A, and Girl B, and they promised, "We'll never tell the teacher."

The next day, Boy A was called in by the teacher and lectured for 30 minutes. None of the four broke the promise.

  • Boy B: After they parted, he forgot about the vase.
  • Boy C: Has a daily routine of telling his father about school, and told him. No one else heard.
  • Girl A: Has a daily routine of telling her mother about school, and told her. No one else heard.
  • Girl B: Told the other student, Girl C, on the phone. Girl C told no one.

Ms. K is a woman and not Boy C's father. Ms. K's child is one of the students at this school, and her daily routine is to hear about school from her child every day. The parents of the other students are not school teachers.

Your question

How did Ms. K know that Boy A was the one who broke the vase?

Case Files

Diagram

The four who promised "never tell the teacher"WhoAfterwardsBForgot about the vase after partingCTold his father about school; nobody overheardA-koTold her mother about school; nobody overheardB-koPhoned C-ko, who told nobodyMs. K is a woman and not C's father. Her child is one of the pupils, andshe hears about school from the child every day.
Diagram

Hints

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

Open hint 1

Did the teacher learn it from someone one of the four told?

Open hint 2

Who is the person Girl A told, "mother"?

Open hint 3

What if the teacher and the person told are the same person?

Solve It

Your question

How did Ms. K know that Boy A was the one who broke the vase?

Choose the answer closest to the truth.

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The Truth

Girl A's mother was Ms. K

The answer is "Girl A's mother was Ms. K."

Boy B did not tell anyone. Boy C told his father, but his father told no one and is not the teacher. Girl C, whom Girl B told, told no one either. If the "mother" that Girl A told was Ms. K herself, then the teacher learned of it even though no one broke the promise.

There is no basis for eavesdropping, and no fact that Boy C's father or Girl C told.

How the news reached the teacherToldReaches theteacher?B: nobodyNoC → his father (not the teacher)NoB-ko → C-ko (tells nobody)NoA-ko → her mother = Ms. KYesA-ko spoke to the teacher herself without realizing, so no promisewas broken.
Diagram of the solution

Key Points

  • Check the routes of transmission in order
  • Notice another name for the same person
  • Assume everyone is honest

Why people get fooled

Because you assume "someone must have broken the promise."

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