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Math Help - number of ways to write

  1. #1
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    number of ways to write

    Kaplan Page 134 # 9

    Five light bulbs that are either on or off are lined up in a row. A certain code uses different combinations of lights turned on to represent different words. If exactly 2 lights must be turned on to represent a word, how many different words can be formed using the 5 light bulbs?

    I'm looking for any fast methods/shortcuts!

    Thanks in advance
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  2. #2
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    5 \choose 2 ????
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  3. #3
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    Thanks. how do you know when to use combination?
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