2000 AMC 12 Problem 16
Problem 16 of 25IntermediateNumber TheoryCounting & Probability
A checkerboard of rows and columns has a number written in each square, beginning in the upper left corner, so that the first row is numbered the second row and so on down the board. If the board is renumbered so that the left column, top to bottom, is the second column 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).
Answer choices
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Solution
The square is numbered originally and after renumbering. Setting these equal gives
The solutions with and are and
These squares hold the numbers and whose sum is
Thus, the correct answer is D.