Maths Olympiad Prep

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, 2018

Number theory Difficulty 4.0 AMC 10/12 Find the answer United States

Problem:

What is the largest factor of 130000130000 that does not contain the digit 00 or 55?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:

If the number is a multiple of 55, then its units digit will be either 00 or 55. Hence, the largest such number must have no factors of 55.

We have 130000=245413130000 = 2^{4} \cdot 5^{4} \cdot 13. Removing every factor of 55, we get that our number must be a factor of 2413=2082^{4} \cdot 13 = 208.

If our number contains a factor of 1313, we cancel the factor of 22 from 208208, 104104, and 5252 until we get 2626. Otherwise, the largest number we can have is 24=162^{4} = 16. We conclude that the answer is 2626.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.