Olympiad Maths Prep

Track / Stage 4 / 288 of 340 #548 of 2000

Problem 548

AMC 12 late, AIME early
Combinatorics Difficulty 5.0 Find the answer

10. The Seventh Story

- Here's a frying pan, - said the King. Will you finally bake me some pretzels or not?
- Without a recipe? - asked the Queen.
- Use your usual recipe, shouted the King impatiently. - Last time your pretzels turned out wonderfully!

!

- I can't, - answered the Queen. - Last time I baked pretzels using a recipe from my cookbook, and it was stolen!

The greatest suspicion fell on the Duchess's cook. The cookbook was indeed found in the Duchess's kitchen. It could have been stolen only by the cook, the Duchess, and the Cheshire Cat.

- The cookbook was stolen by the Cheshire Cat! - declared the Duchess at the trial.
- Oh yes, I did steal the cookbook! - agreed the Cheshire Cat with a smile.
- I didn't steal the cookbook! - asserted the cook.

It turned out that the one who stole the cookbook was lying, and at least one of the other accused told the truth.

Who stole the cookbook?

Official solution

10. The seventh story. The Cheshire Cat could not have stolen the cookbook, for in that case the thief would have been telling the truth. Therefore, the Cheshire Cat did not steal the cookbook (and both the Cat and the Duchess lied at the trial). If the cook had stolen the cookbook, then all three accused would have been lying, which contradicts the conditions of the problem. Therefore, the cookbook was stolen by the Duchess (so the Duchess is lying, the Cheshire Cat is lying, and the cook always tells the truth only).

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.