Difference between revisions of "2000 AMC 12 Problems/Problem 16"

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== See also ==
 
== See also ==
{{AMC12 box|year=2000|num-b=8|num-a=10}}
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[[Category:Introductory Number Theory Problems]]
 
[[Category:Introductory Number Theory Problems]]

Revision as of 20:04, 4 January 2008

Problem

A checkerboard of $13$ rows and $17$ columns has a number written in each square, beginning in the upper left corner, so that the first row is numbered $1,2,\ldots,17$, the second row $18,19,\ldots,34$, and so on down the board. If the board is renumbered so that the left column, top to bottom, is $1,2,\ldots,13,$, the second column $14,15,\ldots,26$ and so on across the board, some squares have the same numbers in both numbering systems. Find the sum of the numbers in these squares (under either system).

$\text {(A)}\ 222 \qquad \text {(B)}\ 333\qquad \text {(C)}\ 444 \qquad \text {(D)}\ 555 \qquad \text {(E)}\ 666$

Solution

Let $(x,y)$ denote the square in row $x \ge 1$ and column $y \ge 1$. Under the first ordering this square would have a value of $17(x-1) + y$. Under the second ordering this square would have a value of $13(y-1) + x$. Equating, $17x-17 + y = 13y-13+x \Longrightarrow 16x = 12y + 4 \Longrightarrow 4x = 3y + 1$. The pairs that fit this equation are $(1,1),(4,5),(7,9),(10,13),(13,17)$; their corresponding values sum up to $1 + 56 + 111 + 166 + 221 = 555\ \mathrm{(D)}$.

See also

2000 AMC 12 (ProblemsAnswer KeyResources)
Preceded by
Problem 15
Followed by
Problem 17
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