Dora bought a bottle of nail polish that was marked down by 20 percent from its original price of $4.50. Including a 9 percent sales tax, what is the final cost of the bottle of nail polish?

First apply the 20% markdown: 20% of $4.50 is $0.90, so the sale price is $4.50 − $0.90 = $3.60. The 9% sales tax is $3.60 × 0.09 = $0.324, making the total $3.924, which rounds to $3.92. The final cost is choice D, $3.92.

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