Genevieve is buying a car audio system that consists of a stereo, speakers, and amplifiers. She is choosing between 5 types of stereos and 2 types of amplifiers. If she has a total of 30 choices, how many types of speakers must there be?

Use the multiplication principle: the total number of systems is the number of stereo choices times the number of speaker choices times the number of amplifier choices. If there are s types of speakers, then 5 × s × 2 = 30. Dividing 30 by 10 gives s = 3, so the correct choice is 3 types of speakers.

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