A factory's Machine A produces 60% of all units with a 3% defect rate, and Machine B produces the remaining 40% with a 5% defect rate.…

Use the law of total probability: weight each machine's defect rate by its share of production. Machine A contributes $0.60 \times 0.03 = 0.018$, and Machine B contributes $0.40 \times 0.05 = 0.020$. Together, $0.018 + 0.020 = 0.038$, which is 3.8%. A common wrong answer is 4, from simply averaging the two defect rates, $\frac{3 + 5}{2}$, which ignores that Machine A makes more of the units. Another is 8, from adding 3% and 5%. A third is 1.8, which counts only Machine A. The overall rate must lie between 3% and 5%, closer to 3%.