2015 AMC 10B Problem 24
Problem 24 of 25HarderGeometryNumber Theory
Aaron the ant walks on the coordinate plane according to the following rules.
He starts at the origin facing to the east and walks one unit, arriving at
For right after arriving at the point if Aaron can turn left and walk one unit to an unvisited point he does that. Otherwise, he walks one unit straight ahead to reach Thus the sequence of points continues in a counterclockwise spiral pattern. What is
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Solution
When Aaron reaches , he has just completed the square spiral containing all grid points with coordinates between and . Therefore
With , this gives . Since , stepping backward along the bottom edge subtracts from the -coordinate, so
Thus, the correct answer is D.