Hi Taiana Melo,
This looks like a PS question (not an IR question), so you should have posted it here:
https://www.beatthegmat.com/problem-solving-f6.html
That having been said, I'll be happy to walk you through it. This is an example of a straight-forward Combinations question. When dealing with Combinations, it often helps to use the Combination Formula:
N! / K!(N-K)! where N is the total number of items and K is the number in the 'sub-group'
Here, we have 12 lightbulbs and we're choosing groups of 2. Since it doesn't matter which lightbulb comes first (choosing bulb A and then bulb B is the SAME as choosing bulb B and then bulb A), the order of the bulbs does NOT matter. THAT is a big 'clue' to use the Combination Formula:
12! / 2!(12-2)! = 12! / 2!10! = (12)(11) / (2)(1) = 132/2 = 66
Final Answer:
B
GMAT assassins aren't born, they're made,
Rich
