Difference between revisions of "2011 AMC 10B Problems/Problem 21"
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Revision as of 12:12, 4 July 2013
Problem
Brian writes down four integers 
whose sum is 
. The pairwise positive differences of these numbers are 
and 
. What is the sum of the possible values for 
?
Solution
The largest difference, 
must be between and 
The smallest difference, 
must be directly between two integers. This also means the differences directly between the other two should add up to 
The only remaining differences that would make this possible are 
and 
However, those two differences can't be right next to each other because they would make a difference of This means 
must be the difference between 
and 
We can express the possible configurations as the lines.
![[asy] unitsize(14mm); defaultpen(linewidth(.8pt)+fontsize(10pt)); dotfactor=4; pair Z1=(0,1), Y1=(1,1), X1=(2,1), W1=(3,1); pair Z4=(4,1), Y4=(5,1), X4=(6,1), W4=(7,1); draw(Z1--W1); draw(Z4--W4); pair[] ps={W1,W4,X1,X4,Y1,Y4,Z1,Z4}; dot(ps); label("$z$",Z1,N); label("$y$",Y1,N); label("$x$",X1,N); label("$w$",W1,N); label("$z$",Z4,N); label("$y$",Y4,N); label("$x$",X4,N); label("$w$",W4,N); label("$1$",(X1--Y1),N); label("$1$",(X4--Y4),N); label("$3$",(Y1--Z1),N); label("$3$",(W4--X4),N); label("$5$",(X1--W1),N); label("$5$",(Y4--Z4),N); [/asy]](http://latex.artofproblemsolving.com/c/e/1/ce15bff994b2e1387b4b56d1de02fde16bfbfc54.png)
If we look at the first number line, you can express 
as 
as 
and 
as 
Since the sum of all these integers equal ,
You can do something similar to this with the second number line to find the other possible value of 
The sum of the possible values of is 
See Also
| 2011 AMC 10B (Problems • Answer Key • Resources) | ||
| Preceded by Problem 20 |
Followed by Problem 22 | |
| 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. 
