Problem 9. (Mary's Random Walk)

Mary lives in a town which has streets and avenues crossing each other at right angles. One day at exactly noon Mary decides to take a walk starting at the intersection of a street and avenue, whereupon she covers one block in every 15 minutes. When she reaches the end of a block, she turns either to the left or right. Show that Mary can return to the starting point only on the hour.

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Last modified on Monday, February 08, 1999