The quantity of ink used to compose a text for the Mathematical Olympiads follows a strange law: in odd years it increases by 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). 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+1 years have passed, then, it will have become 23⋅(65⋅23)k. Requiring that this quantity be greater than 3, after a few simple steps we find ourselves having to find the minimum k for which (45)k>2 but for k=3 we get 64125<64128=2, while for k=4 we find 256625 which is clearly greater than 2. We will then be in the 2⋅4+1-th year, that is, the ninth.
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Source: MathNet,
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