Question

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?

Answer 1

i think you are being asked how many ways you can choose 2 bulbs out of 5, i.e asked to compute
\[\binom{5}{2}=\frac{5\times 4}{2}\]