2018 AIME I Problems/Problem 11
Find the least positive integer such that when
is written in base
, its two right-most digits in base
are
.
Contents
Solutions
Modular Arithmetic Solution- Strange (MASS)
Note that the given condition is equivalent to and
. Because
, the desired condition is equivalent to
and
.
If , one can see the sequence
so
.
Now if , it is harder. But we do observe that
, therefore
for some integer
. So our goal is to find the first number
such that
. In other words, the
. It is not difficult to see that the smallest
, so ultimately
. Therefore,
.
The first satisfying both criteria is thus
.
-expiLnCalc
Solution 2
Note that Euler's Totient Theorem would not necessarily lead to the smallest and that in this case that
is greater than
.
We wish to find the least such that
. This factors as
. Because
, we can simply find the least
such that
and
.
Quick inspection yields and
. Now we must find the smallest
such that
. Euler's gives
. So
is a factor of
. This gives
. Some more inspection yields
is the smallest valid
. So
and
. The least
satisfying both is
. (RegularHexagon)
Solution 3 (BigBash)
Listing out the powers of , modulo
and modulo
, we have:
The powers of repeat in cycles of
an
in modulo
and modulo
, respectively. The answer is
.
See Also
2018 AIME I (Problems • Answer Key • Resources) | ||
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Followed by Problem 12 | |
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