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Difference between revisions of "2014 AMC 10B Problems/Problem 10"

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==Solution==
 
==Solution==
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Note from the addition of the last digits that <math>A+B=D\text{ or }D+10</math>. In the latter case we must have that <math>1+C+D=D\text{ or }10+D</math>, implying that <math>C=9</math>. In the addition of the third digits, we then have <math>1+B+A=D\text{ or }D+10</math>, a contradiction from our assumption that <math>A+B=10</math>. Thus <math>A+B=D</math>.
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This then implies that <math>C+D=D</math>, or <math>C=0</math>. Note that all of the remaining equalities are now satisfied: <math>A+B=D, B+C=B,</math> and <math>B+A=D</math>. Thus, if we have some <math>A,B,D</math> such that <math>A+B=D</math> then the addition will be satisfied. Since the digits must be distinct, the smallest possible value of <math>D</math> is <math>1+2=3</math>, and the largest possible value is <math>9</math>. Any of these values can be obtained by taking <math>A=1,B=D-1</math>. Thus we have that <math>3\le D\le9</math>, so the number of possible values is <math>\boxed{\textbf{(C) }7</math>
  
 
==See Also==
 
==See Also==
 
{{AMC10 box|year=2014|ab=B|num-b=9|num-a=11}}
 
{{AMC10 box|year=2014|ab=B|num-b=9|num-a=11}}
 
{{MAA Notice}}
 
{{MAA Notice}}

Revision as of 18:35, 20 February 2014

Problem

In the addition shown below $A$, $B$, $C$, and $D$ are distinct digits. How many different values are possible for $D$?

\[\begin{array}[t]{r} ABBCB \\ + \ BCADA \\ \hline DBDDD \end{array}\]

Solution

Note from the addition of the last digits that $A+B=D\text{ or }D+10$. In the latter case we must have that $1+C+D=D\text{ or }10+D$, implying that $C=9$. In the addition of the third digits, we then have $1+B+A=D\text{ or }D+10$, a contradiction from our assumption that $A+B=10$. Thus $A+B=D$.

This then implies that $C+D=D$, or $C=0$. Note that all of the remaining equalities are now satisfied: $A+B=D, B+C=B,$ and $B+A=D$. Thus, if we have some $A,B,D$ such that $A+B=D$ then the addition will be satisfied. Since the digits must be distinct, the smallest possible value of $D$ is $1+2=3$, and the largest possible value is $9$. Any of these values can be obtained by taking $A=1,B=D-1$. Thus we have that $3\le D\le9$, so the number of possible values is $\boxed{\textbf{(C) }7$ (Error compiling LaTeX. Unknown error_msg)

See Also

2014 AMC 10B (Problems • Answer Key • Resources)
Preceded by
Problem 9 		Followed by
Problem 11
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

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