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March 25, 2017

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In the English alphabet, there are 7 capital letters that look the same after being rotated 180∘. Those letters are H, I, N, O, S, X, and Z. How many different 3 letter "words" can be made from these letters such that when the word is rotated 180∘, the resulting word is the same?

  • geometry - ,

    Clearly the 1st and 3rd letter must be the same, but the middle can be any of the 7 letters

    number of ways = 7 x 7 x 1 = 49

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