One approach is to use the Double Matrix Method. This technique can be used for questions featuring a population in which each member has two characteristics associated with it (aka overlapping sets questions).
Here, we have a population of people, and the two characteristics are:
- on committee A or not on committee A
- on committee B or not on committee B

Aside: We can also use Venn diagrams and formulae to solve overlapping sets questions. However, as difficulty levels increase, it becomes harder to apply those other approaches, whereas the Double Matrix Method works every time.

From the given information, we can set up our diagram as follows:

Notice that the two yellow boxes represent people serving on only 1 committee.
So to answer the question, we need to find the sum of those two yellow boxes

Since we already know the sum of the two boxes in the left-hand column, and the sum of the two boxes in the top row, we can complete our diagram as follows


So the number of people serving on only one committee = 5 + 4 = 9

Answer: B

To learn more about the Double Matrix Method, watch this video:


Then you can try solving these two questions:
- https://gre.myprepclub.com/forum/qotd-11-o ... -2660.html
- https://gre.myprepclub.com/forum/on-a-cert ... 13470.html