2006 AMC 12B Problems/Problem 18
An object in the plane moves from one lattice point to another. At each step, the object may move one unit to the right, one unit to the left, one unit up, or one unit down. If the object starts at the origin and takes a ten-step path, how many different points could be the final point?
Let the starting point be . After steps we can only be in locations where . Additionally, each step changes the parity of exactly one coordinate. Hence after steps we can only be in locations where is even. It can easily be shown that each location that satisfies these two conditions is indeed reachable.
Once we pick , we have valid choices for , giving a total of possible positions.
moves results in a lot of possible endpoints, so we try small cases first.
If the object only makes move, it is obvious that there are only 4 possible points that the object can move to. If the object makes moves, it can move to , , , , , , as well as , for a total of moves. If the object makes moves, it can end up at , , , , , , , , , etc. for a total of moves.
At this point we can guess that for n moves, there are different endpoints. Thus, for 10 moves, there are endpoints, and the answer is .
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