Math and critical thinking challenges like this one are an educational activity for practicing children’s math skills. These challenges are also a fun way for adults to test their math knowledge.

## Challenge:

There are 21 students in a class. Their Math teacher asked them this question: “How many of other students in this class have the same name as you?” The teacher asked the students to write their answers in a piece of paper. Then the teacher gathered the answers and removed repeated answers. What is the maximum number of answers that the teacher kept?

**A-** 0

**B-** 1

**C-** 2

**D-** 20

**E-** 21

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The correct answer is B.

Let’s assume that all of the students have the name John. So, to answer the questions, all of them have written 20. In this case, the teacher kept none of the answers.

If none of the students have the same name, they all answer 0. And the teacher won’t keep any answer.

Let’s assume that only one of them has a different name. Then, in this case, 20 students wrote 19 and one wrote 0. The teacher kept the last one.

The maximum number of answers that the teacher kept was one.