Antonio is buying a stereo, speakers, and amplifiers for his car audio system. He is choosing among 4 models of stereos and 3 styles of speakers. If he has 36 different choices, how many kinds of amplifiers must there be?

For each stereo, Antonio can choose from 3 speaker styles and any kind of amplifier. The total number of combinations is 4 × 3 × the number of amplifier kinds, so 12 × the number of amplifier kinds = 36. Dividing 36 by 12 gives 3, so the correct choice is 3 kinds of amplifiers.

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