2022 AMC 10B Problems/Problem 17

Problem

One of the following numbers is not divisible by any prime number less than 10. Which is it?

Solution

For A, modulo 3, $\begin{align*} 2^{606} - 1 & \equiv (-1)^{606} - 1 \\ & \equiv 1 - 1 \\ & \equiv 0 . \end{align*}$

Thus, $2^{606} - 1$ is divisible by 3.

For B, modulo 5, $\begin{align*} 2^{606} + 1 & \equiv 2^{{\rm Rem} ( 606, \phi(5) )} + 1 \\ & \equiv 2^{{\rm Rem} ( 606, 4 )} + 1 \\ & \equiv 2^2 + 1 \\ & \equiv 0 . \end{align*}$

Thus, $2^{606} + 1$ is divisible by 5.

For D, modulo 3, $\begin{align*} 2^{607} + 1 & \equiv (-1)^{607} + 1 \\ & \equiv - 1 + 1 \\ & \equiv 0 . \end{align*}$

Thus, $2^{607} + 1$ is divisible by 3.

For E, module 5, $\begin{align*} 2^{607} + 3^{607} & \equiv 2^{607} + (-2)^{607} \\ & \equiv 2^{607} - 2^{607} \\ & \equiv 0 . \end{align*}$

Thus, $2^{607} + 3^{607}$ is divisible by 5.

Therefore, the answer is \boxed{\textbf{(C) $2^{607} - 1$ }}.

~Steven Chen (Professor Chen Education Palace, www.professorchenedu.com)

Video Solution

https://youtu.be/YF3HPVcVGZk

~Steven Chen (Professor Chen Education Palace, www.professorchenedu.com)

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2022 AMC 10B Problems/Problem 17

Problem

Solution

Video Solution