# 2000 AMC 12 Problems/Problem 16

## 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

Index the rows with $i = 1, 2, 3, ..., 13$ Index the columns with $j = 1, 2, 3, ..., 17$

For the first row number the cells $1, 2, 3, ..., 17$ For the second, $18, 19, ..., 34$ and so on

So the number in row = $i$ and column = $j$ is $f(i, j) = 17(i-1) + j = 17i + j - 17$

Similarly, numbering the same cells columnwise we find the number in row = $i$ and column = $j$ is $g(i, j) = i + 13j - 13$

So we need to solve $f(i, j) = g(i, j)$ $17i + j - 17 = i + 13j - 13$ $16i = 4 + 12j$ $4i = 1 + 3j$ $i = (1 + 3j)/4$

We get $(i, j) = (1, 1), f(i, j) = g(i, j) = 1$ $(i, j) = (4, 5), f(i, j) = g(i, j) = 56$ $(i, j) = (7, 9), f(i, j) = g(i, j) = 111$ $(i, j) = (10, 13), f(i, j) = g(i, j) = 166$ $(i, j) = (13, 17), f(i, j) = g(i, j) = 221$ $\boxed{D}$ $555$

## Video Solution

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