Difference between revisions of "2017 AMC 10B Problems/Problem 1"

Revision as of 10:09, 26 July 2017

Problem

Mary thought of a positive two-digit number. She multiplied it by $3$ and added $11$ . Then she switched the digits of the result, obtaining a number between $71$ and $75$ , inclusive. What was Mary's number?

$\textbf{(A)}\ 11\qquad\textbf{(B)}\ 12\qquad\textbf{(C)}\ 13\qquad\textbf{(D)}\ 14\qquad\textbf{(E)}\ 15$

Solution

Solution 1

Let her $2$ -digit number be $x$ . Multiplying by $3$ makes it a multiple of $3$ , meaning that the sum of its digits is divisible by $3$ . Adding on $11$ increases the sum of the digits by $1+1 = 2,$ and reversing the digits keeps the sum of the digits the same; this means that the resulting number must be $2$ more than a multiple of $3$ . There are two such numbers between $71$ and $75$ : $71$ and $74.$ 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 $71,$ we reverse the digits, resulting in $17.$ Subtracting $11$ , we get $6.$ We can already see that dividing this by $3$ will not be a two-digit number, so $71$ does not meet our requirements. $\[\]$ Therefore, the answer must be the reversed steps applied to $74.$ We have the following: $\[\]$ $74\rightarrow47\rightarrow36\rightarrow12$ $\[\]$ Therefore, our answer is $\boxed{\bold{(B)} 12}$ .

Solution 2

Working backwards, we reverse the digits of each number from $71$ ~ $75$ and subtract $11$ from each, so we have $6, 16, 26, 36, 46$ The only numbers from this list that are divisible by $3$ are $6$ and $36$ . We divide both by $3$ , yielding $2$ and $12$ . Since $2$ is not a two-digit number, the answer is $\boxed{\textbf{(B)}\ 12}$ .

@@ Line 5: / Line 5: @@
 <math>\textbf{(A)}\ 11\qquad\textbf{(B)}\ 12\qquad\textbf{(C)}\ 13\qquad\textbf{(D)}\ 14\qquad\textbf{(E)}\ 15</math>
-==Solution 1==
+==Solution==
+===Solution 1===
 Let her <math>2</math>-digit number be <math>x</math>. Multiplying by <math>3</math> makes it a multiple of <math>3</math>, meaning that the sum of its digits is divisible by <math>3</math>. Adding on <math>11</math> increases the sum of the digits by <math>1+1 = 2,</math> and reversing the digits keeps the sum of the digits the same; this means that the resulting number must be <math>2</math> more than a multiple of <math>3</math>. There are two such numbers between <math>71</math> and <math>75</math>: <math>71</math> and <math>74.</math> 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:
 <cmath></cmath>
@@ Line 14: / Line 16: @@
 <math>74\rightarrow47\rightarrow36\rightarrow12</math>
 <cmath></cmath>
-Therefore, our answer is <math>\boxed{\bold{(B)} 12.}</math>
+Therefore, our answer is <math>\boxed{\bold{(B)} 12}</math>.
+===Solution 2===
+Working backwards, we reverse the digits of each number from <math>71</math>~<math>75</math> and subtract <math>11</math> from each, so we have
+<cmath>6, 16, 26, 36, 46</cmath>
+The only numbers from this list that are divisible by <math>3</math> are <math>6</math> and <math>36</math>. We divide both by <math>3</math>, yielding <math>2</math> and <math>12</math>. Since <math>2</math> is not a two-digit number, the answer is <math>\boxed{\textbf{(B)}\ 12}</math>.
 ==See Also==
 {{AMC10 box|year=2017|ab=B|before=First Problem|num-a=2}}
 {{MAA Notice}}

2017 AMC 10B (Problems • Answer Key • Resources)
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