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The probability of buying a faulty generator is 4 out of 50, or 4/50 = 0.08.
If the buyer buys 200 generators, the expected number of faulty generators is 0.08 * 200 = 16.
Therefore, the probability of the buyer buying a faulty generator out of the 200 he buys is 16/200, or 0.08 or 8%.