This problem should be solved following way

There are 2 products "P" and "Q", and we have to answer what fraction of people do not select both the products.

Option 1- 1/3 of people select only P and not Q

So, suppose we have 90 people responded to survey then 1/3 of 90 = 30 people select only product P

But, this option does not tells us anything about Q, so not sufficient to answer the question.

Option 2- 1/2 people select product Q ....this includes people who selected inly product Q and people who selected

both product P and Q i.e. P intersection Q

Therefore if 90 people responded to survey 45 people selected product Q, but it does not tells us how

many people select only product p

Now, it we combine the options it gives us value of A U B i.e if we have 90 people on the survey 30 selected product P and 45 selected product Q along with P intersection Q

Therefore P U Q = 30 + 45 = 75

Therefore (P U Q)' = 90 - 75 = 15

Hence fraction of people did not select any product = 15/90

Hence we get the answer by taking both the options together... hence answer "C"

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