Maths Olympiad Prep

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Algebra Difficulty 4.9 AIME Find the answer Italy

Problem:

The quantity of ink used to compose a text for the Mathematical Olympiads follows a strange law: in odd years it increases by 50%50\% compared to the previous year, in even years it decreases by one sixth (again compared to the previous year). In how many years will it be, for the first time, at least triple that of 2012?

Pick one

Solution

Solution:

The answer is (E)\mathbf{( E )}. Certainly the quantity of ink will exceed triple that of 2012 for the first time in an odd year, since in even years it decreases. If 2k+12k+1 years have passed, then, it will have become
32(5632)k. \frac{3}{2} \cdot \left(\frac{5}{6} \cdot \frac{3}{2}\right)^{k}.
Requiring that this quantity be greater than 33, after a few simple steps we find ourselves having to find the minimum kk for which
(54)k>2 \left(\frac{5}{4}\right)^{k} > 2
but for k=3k=3 we get 12564<12864=2\frac{125}{64} < \frac{128}{64} = 2, while for k=4k=4 we find 625256\frac{625}{256} which is clearly greater than 22. We will then be in the 24+12 \cdot 4 + 1-th year, that is, the ninth.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from it; metadata (topic, difficulty) added by this project.