2022 AMC 10A Problem 12

Question

On Halloween

31

children walked into the principal’s office asking for candy. They can be classified into three types: Some always lie; some always tell the truth; and some alternately lie and tell the truth. The alternaters arbitrarily choose their first response, either a lie or the truth, but each subsequent statement has the opposite truth value from its predecessor. The principal asked everyone the same three questions in this order.

“Are you a truth-teller?” The principal gave a piece of candy to each of the

22

children who answered yes.

“Are you an alternater?” The principal gave a piece of candy to each of the

15

children who answered yes.

“Are you a liar?” The principal gave a piece of candy to each of the

9

children who answered yes.

How many pieces of candy in all did the principal give to the children who always tell the truth?

7

12

21

27

31

Answer: A

Concepts:truth-tellers and liars logical deduction

Difficulty rating: 1950

Small hint:

Separate alternaters by whether they start with truth or lie

Big hint:

The first two yes-counts are enough to determine the number of truth-tellers

Solution:

For the first question, the truth-tellers will respond yes, the liars will respond yes, and the alternaters who decided to lie first will say yes. The alternaters who decide to tell the truth first will say no. Denote this as

22 = t + l + a_l.

For the second questions, the liars will respond yes, and the alternaters who decided to lie first will say yes (they are forced to tell the truth for this question). The truth-tellers will respond no, and the alternaters who told the truth first would lie this round, responding no. Denote this as

15 = l + a_l.

From this, we get that

t = 7.

The principal only gives candy to children who always tell the truth in the first round, therefore only giving them

7

candies total.

Thus, A is the correct answer.

2022 AMC 10A Problem 12

Answer: A

Difficulty rating: 1950

Small hint:

Big hint:

Solution:

Problem 12 in other years