2017 AMC 8 Problems/Problem 7

Problem

Let $Z$ be a 6-digit positive integer, such as 247247, whose first three digits are the same as its last three digits taken in the same order. Which of the following numbers must also be a factor of $Z$ ?

$\textbf{(A) }11\qquad\textbf{(B) }19\qquad\textbf{(C) }101\qquad\textbf{(D) }111\qquad\textbf{(E) }1111$

Solution 1 (detailed explanation)

To check, if a number is divisible by 19, take its unit digit and multiply it by 2, then add the result to the rest of the number, and repeat this step until the number is reduced to two digits. If the result is divisible by 19, then the original number is also divisible by 19. Or we could just try to divide the example number the problem gave us by 19.

After we got 19 eliminated, we can see that the other options have a lot of 1's in them. The divisibility rule for 11 is add alternating digits up, then take the difference of them. The example number works like that. If we add variables, ABCDEF to make number ABCABC, we can see that (A+C+B) - (B+A+C) = 0. Which is divisible by 11, so our answer choice is $\boxed{\textbf{(A)}\ 11}$ .

by: CHECKMATE2021 (edited by CHECKMATE2021)

Solution 2

We are given one of the numbers that can represent $Z$ , so we can just try out the options to see which one is a factor of $247247$ . We get $\boxed{\textbf{(A)}\ 11}$ .

Solution 3

To find out when a number is divisible by 11, place plus and minus signs alternatively in front of every digit, then calculate the result. If this result is divisible by 11 (including 0), the number is divisible by 11; otherwise, the number isn’t divisible by 11. In this case, $+2-4+7-2+4-7=0$ . Because the result is 0, the number 247247 is divisible by 11 and so we get $\boxed{\textbf{(A)}\ 11}$ .

--LarryFlora

Solution 4( similar to solution 1)

Similar to solution 1, let $Z=ABCABC$ . To prove it is divisible by 11, we can compute its alternating sum, which is $A-B+C-A+B-C=0$ , which is divisible by 11. Therefore, the answer is $\boxed{\textbf{(A)}\ 11}$ .

~PEKKA

Solution 5

We can find that all numbers like $Z$ are divisible by 1001. 1001 is divisible by 11 because when we divide it, we get a whole number. So, the answer is $\boxed{\textbf{(A)}\ 11}$ .

~AfterglowBlaziken

Video Solution (CREATIVE THINKING!!!)

https://youtu.be/WVHPSVOXE4I

~Education, the Study of Everything

Video Solution by OmegaLearn

https://youtu.be/At4w8uylvv8?t=99 https://youtu.be/7an5wU9Q5hk?t=647

Video Solution

https://youtu.be/-saKu3lLU0U

~savannahsolver

2017 AMC 8 (Problems • Answer Key • Resources)
Preceded by Problem 6	Followed by Problem 8
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All AJHSME/AMC 8 Problems and Solutions

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