The error is that the numbers are not in order from least to greatest. To find the median of data you need to first put the numbers in order from least to greatest.
5/20 students in Alessandro's class live in Oakdale, which can be simplified to 1/4. 1/4 in decimal form is 0.25. Now to convert it to a percentage, you just multiply that by 100. So 25% of students in Alessandro's class live in Oakdale.
