You will be given a definition of a task first, then some input of the task.
This task is about writing a correct answer for the reading comprehension task. Based on the information provided in a given passage, you should identify the shortest continuous text span from the passage that serves as an answer to the given question. Avoid answers that are incorrect or provides incomplete justification for the question.

Passage: The total adult literacy rate is 99 percent. Portuguese primary school enrollments are close to 100 percent. According to the OECD's Programme for International Student Assessment (PISA) 2009, the average Portuguese 15-year-old student, when rated in terms of reading literacy, mathematics and science knowledge, is placed at the same level as those students from the United States, Sweden, Germany, Ireland, France, Denmark, United Kingdom, Hungary and Taipei, with 489 points (493 is the average). Over 35% of college-age citizens (20 years old) attend one of the country's higher education institutions (compared with 50% in the United States and 35% in the OECD countries). In addition to being a destination for international students, Portugal is also among the top places of origin for international students. All higher education students, both domestic and international, totaled 380,937 in 2005. Question: What percentage of college-age citizens attend a higher education institution in the United States?
Output:
50%