It can be 0 and then 42% of the same would also be 0 (number of females). Where does it say in the question that number of females have to be non-zero. All we know is that females = 0.42*a and this equation can be valid at both a = 0 and a = 50 without contradicting anything given in the question.(2) Of the students who own a car, 42% are female.
The phrase 'Of the students who own a car' already tells you that some students own a car, so it is not a possibility that 0 students own a car. Further, if I were to tell you that 0 people own a car, and I asked you what percentage of the students who own a car are female, you would not say "42%". It would be meaningless to assign a percentage in that case. The GMAT would never tell you '42% of people own a car' and then make the 'trap' that 0 people might own a car, because most test takers would, very reasonably, assume the number of car owners needs to be non-zero.
So if you see a phrase like '1/3 of the people in North America who own a computer are Canadian' or '45% of the marbles in a bag are green', you can certainly assume that the number of North Americans who own a computer, or the number of marbles in the bag, is not zero.
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