2000 AMC 12 Problem 16

Attempt Problem 16 of the 2000 AMC 12 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2000 AMC 12 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

16.

A checkerboard of 1313 rows and 1717 columns has a number written in each square, beginning in the upper left corner, so that the first row is numbered 1,1, 2,2, …,\ldots, 17,17, the second row 18,18, 19,19, …,\ldots, 34,34, and so on down the board. If the board is renumbered so that the left column, top to bottom, is 1,1, 2,2, …,\ldots, 13,13, the second column 14,14, 15,15, …,\ldots, 2626 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).
222222
333333
444444
555555
666666
Answer: D
Concepts:Diophantine Equationsystematic listing
Small hint:
The square in row m,m, column nn is numbered 17(m−1)+n17(m - 1) + n originally and 13(n−1)+m13(n - 1) + m after
Big hint:
Setting the two expressions equal gives 4m−3n=14m - 3n = 1; find all valid (m,n)(m, n)
Solution:
The square (m,n)(m, n) is numbered 17(m−1)+n17(m - 1) + n originally and 13(n−1)+m13(n - 1) + m after renumbering. Setting these equal gives 4m−3n=1. 4m - 3n = 1.
The solutions with 1≤m≤131 \le m \le 13 and 1≤n≤171 \le n \le 17 are (1,1),(1, 1), (4,5),(4, 5), (7,9),(7, 9), (10,13),(10, 13), and (13,17).(13, 17).
These squares hold the numbers 1,56,111,166,1, 56, 111, 166, and 221,221, whose sum is 555.555.
Thus, the correct answer is D.
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