2022 AMC 10 B

Complete problem set with solutions and individual problem pages

Problem 6 Easy

How many of the first ten numbers of the sequence 121, 11211, 1112111,.\dots. are prime numbers?

  • A.

    0

  • B.

    1

  • C.

    2

  • D.

    3

  • E.

    4

Answer:A

Enumerate the terms and find the pattern:

\begin{aligned} 121 & =11 \cdot 11, \\ 11211 & =101 \cdot 111, \\ 1112111 & =1001 \cdot 1111, \end{aligned}, and so on. Therefore, no numbers in the sequence would be prime.

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