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The problem is:

Given a number, find out if this number could be the beginning digits of a 2 exponent & if it can output which power it can be.

As an example given 2 there is answer $8$ which $2 ^ 8 = 256$ which begins with $2$ -> the number given.

Thanks.

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closed as unclear what you're asking by Shailesh, JMoravitz, Matthew Conroy, Rohan, Claude Leibovici Jan 25 '17 at 6:31

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  • $\begingroup$ what? ${}{}{}{}{}$ $\endgroup$ – Jorge Fernández Hidalgo Jan 25 '17 at 3:50
  • $\begingroup$ Do you mean the following: Given a number $n$ find if there is a value $k$ such that the last digits of $2^k$ form the number $n$? $\endgroup$ – Jorge Fernández Hidalgo Jan 25 '17 at 3:51
  • $\begingroup$ no, I meant the first digits. sorry for ambiguity :) $\endgroup$ – Alireza Jan 25 '17 at 3:54
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Such a number always exists,suppose $n$ is the given number, you just need to find an integer $a$ such that the fractional part of $a\log_{10}2$ approximates the fractional part of $\log_{10}(n)$ from above sufficiently well.

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  • $\begingroup$ could you please explain this in an example? $\endgroup$ – Alireza Jan 25 '17 at 4:05

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