Yi is running for class president. To try to predict how people will vote, she wants to take a poll with a 90% confidence level (z*-score of 1.645) and an estimated margin of error of 4%.

When the proportion is unknown, use 0.50 as the estimate because it gives the largest required sample size. The sample-size formula is n = (z*)²p(1 − p)/E², so n = (1.645)²(0.50)(0.50)/(0.04)² ≈ 422.82. Round up to ensure the margin of error is no more than 4%, giving 423 students, choice C.

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