Difference between revisions of "2017 AMC 10B Problems/Problem 1"
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==Solution 3== | ==Solution 3== | ||
You can just plug in the numbers to see which one works. When you get to <math>12</math>, you multiply by <math>3</math> and add <math>11</math> to get <math>47</math>. When you reverse the digits of <math>47</math>, you get <math>74</math>, which is within the given range. Thus, the answer is <math>\boxed{\textbf{(B)}\ 12}</math>. | You can just plug in the numbers to see which one works. When you get to <math>12</math>, you multiply by <math>3</math> and add <math>11</math> to get <math>47</math>. When you reverse the digits of <math>47</math>, you get <math>74</math>, which is within the given range. Thus, the answer is <math>\boxed{\textbf{(B)}\ 12}</math>. | ||
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+ | ==Solution 4 Fastest Way== | ||
+ | Let x be the original number. The last digit of <math>3x+11</math> must be <math>7</math> so the last digit of <math>3x</math> must be <math>6</math>. The only answer choice that satisfies this is <math>\boxed{\textbf{(B)}\ 12}</math>. | ||
==Video Solution== | ==Video Solution== |
Revision as of 08:23, 4 November 2021
Contents
Problem
Mary thought of a positive two-digit number. She multiplied it by and added
. Then she switched the digits of the result, obtaining a number between
and
, inclusive. What was Mary's number?
Solution 1
Let her -digit number be
. Multiplying by
makes it a multiple of
, meaning that the sum of its digits is divisible by
. Adding on
increases the sum of the digits by
(we can ignore numbers such as
) and reversing the digits keeps the sum of the digits the same; this means that the resulting number must be
more than a multiple of
. There are two such numbers between
and
:
and
Now that we have narrowed down the choices, we can simply test the answers to see which one will provide a two-digit number when the steps are reversed:
For
we reverse the digits, resulting in
Subtracting
, we get
We can already see that dividing this by
will not be a two-digit number, so
does not meet our requirements.
Therefore, the answer must be the reversed steps applied to
We have the following:
Therefore, our answer is
.
Solution 2
Working backwards, we reverse the digits of each number from ~
and subtract
from each, so we have
The only numbers from this list that are divisible by
are
and
. We divide both by
, yielding
and
. Since
is not a two-digit number, the answer is
.
Solution 3
You can just plug in the numbers to see which one works. When you get to , you multiply by
and add
to get
. When you reverse the digits of
, you get
, which is within the given range. Thus, the answer is
.
Solution 4 Fastest Way
Let x be the original number. The last digit of must be
so the last digit of
must be
. The only answer choice that satisfies this is
.
Video Solution
~savannahsolver
Video Solution by TheBeautyofMath
~IceMatrix
See Also
2017 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by First Problem |
Followed by Problem 2 | |
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 10 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.