Thread: 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

????

Thanks. how do you know when to use combination?