Darn 2 AM Number Theory
by shiningsunnyday, Sep 1, 2016, 6:06 PM
2016 AMC 12B P22 wrote:
For a certain positive integer
less than
, the decimal equivalent of
is
, a repeating decimal of period of
, and the decimal equivalent of
is
, a repeating decimal of period
. Find 









Solution
By definition of the period of a repeating decimal, we must have the order of
be
and the order of
be
Thus we obtain: 

Since
there must exist some
such that
otherwise we would have
violating the order.
Similarly we must have a prime factor of
divide into
Since
is a prime,
Thus
for some 
We have three cases now.
If

If

If
there exists no
in the range.
Thus the only possibilities are
A quick prime-factorization check yields that only
divides into
and thus is the only viable answer.






Since




Similarly we must have a prime factor of






We have three cases now.
If


If


If


Thus the only possibilities are



Tidbit
So I was reading Binomial-Theorem's announcement and saw him reference this problem, which was really straightforward but nice for me after completing all the theory section of 104.
These days I'm mainly just doing problems I feel are interesting - and not forcing myself to concentrate excessively and for long durations of time. In fact, I'm probably going to significantly dial down my training rigor from now until October break due to symptoms of burnout.
So mods should take this time and post good problems.
Darn darn what am I still doing at 2 AM.[/quote]
These days I'm mainly just doing problems I feel are interesting - and not forcing myself to concentrate excessively and for long durations of time. In fact, I'm probably going to significantly dial down my training rigor from now until October break due to symptoms of burnout.
So mods should take this time and post good problems.

Darn darn what am I still doing at 2 AM.[/quote]
This post has been edited 1 time. Last edited by shiningsunnyday, Sep 2, 2016, 1:53 AM