# number of ways to write

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• December 4th 2008, 05:37 PM
fabxx
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
• December 4th 2008, 05:55 PM
Plato
$5 \choose 2$????
• December 4th 2008, 05:58 PM
fabxx
Thanks. how do you know when to use combination?