189.A Hundred-Storey House of Cards
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A house of cards is built so that the top storey is a single A-frame of 2 cards, and each storey below adds one more A-frame plus the horizontal cards needed to support the storey above. How many cards does a 100-storey house need?
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Count the two kinds of card separately: the leaning pairs and the flat cards between storeys. Both counts are simple sums.
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