Difference between revisions of "2014 AMC 12A Problems/Problem 23"
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So, the answer is <math>0+0+0+1+0+2+0+3+...+9+7+9+9=2\cdot10\cdot\frac{9\cdot10}{2}-(9+8)</math> or <math>\boxed{\textbf{(B)}\ 883}</math>. | So, the answer is <math>0+0+0+1+0+2+0+3+...+9+7+9+9=2\cdot10\cdot\frac{9\cdot10}{2}-(9+8)</math> or <math>\boxed{\textbf{(B)}\ 883}</math>. | ||
− | + | == Video Solution by Richard Rusczyk == | |
https://artofproblemsolving.com/videos/amc/2014amc12a/382 | https://artofproblemsolving.com/videos/amc/2014amc12a/382 | ||
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==See Also== | ==See Also== | ||
{{AMC12 box|year=2014|ab=A|num-b=22|num-a=24}} | {{AMC12 box|year=2014|ab=A|num-b=22|num-a=24}} | ||
{{MAA Notice}} | {{MAA Notice}} |
Revision as of 20:05, 7 December 2020
Contents
Problem
The fraction where
is the length of the period of the repeating decimal expansion. What is the sum
?
Solution 1
the fraction can be written as
.
similarly the fraction
can be written as
which is equivalent to
and we can see that for each
there are
combinations so the above sum is equivalent to:
we note that the sequence starts repeating at
yet consider
so the decimal will go from 1 to 99 skipping the number 98
and we can easily compute the sum of the digits from 0 to 99 to be
subtracting the sum of the digits of 98 which is 17 we get
Solution 2
So, the answer is or
.
There are two things to notice here. First, has a very simple and unique decimal expansion, as shown. Second, for
to itself produce a repeating decimal,
has to evenly divide a sufficiently extended number of the form
. This number will have
ones (197 digits in total), as to be divisible by
and
. The enormity of this number forces us to look for a pattern, and so we divide out as shown. Indeed, upon division (seeing how the remainders always end in "501" or "601" or, at last, "9801"), we find the repeating part
. If we wanted to further check our pattern, we could count the total number of digits in our quotient (not counting the first three): 195. Since
, multiplying by it will produce either
or
extra digits, so our quotient passes the test.
Solution 3 (Fastest)
Simpler version of the above solution
Note that the 1 has to be carried when you get to 100 so the 99 becomes 00 and the 1 gets carried again to 98 which becomes 99.
So, the answer is or
.
Video Solution by Richard Rusczyk
https://artofproblemsolving.com/videos/amc/2014amc12a/382
See Also
2014 AMC 12A (Problems • Answer Key • Resources) | |
Preceded by Problem 22 |
Followed by Problem 24 |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | |
All AMC 12 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.