Machine A makes of a factory's items, of which are defective. Machine B makes the other , of which are defective. What is the probability that a randomly chosen item is defective?

Answer and explanation

Correct answer: D.

  1. Step 1: Defective from A: . Defective from B: . Step 2. The events are disjoint, so the total is . **Answer: . Why not the others:** : This is the simple average of and , ignoring the weights. : This adds the two defect rates. : This is only the contribution of halved.

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