To find the percentage of quiz questions that Sean answered correctly, you can use the formula:
\[ \text{Percentage} = \left( \frac{\text{Number of Correct Answers}}{\text{Total Number of Questions}} \right) \times 100 \]
In this case, Sean answered 18 questions correctly out of a total of 20 questions. Plugging in those numbers:
\[ \text{Percentage} = \left( \frac{18}{20} \right) \times 100 \]
\[ \text{Percentage} = 0.9 \times 100 = 90% \]
Therefore, Sean answered 90% of the quiz questions correctly.