50% of 20,000 is the amount of students that were accepted. That makes 10,000 students accepted. 25% of 10,000 is the amount enrolled. 25% is 1/4. So 1/4 of 10,000 is 2,500. So 2,500 students enrolled in that university.
The probability that Alexa spins an orange is zero. Since there is no orange on the spinner, there is no way for her to get an orange. Hope this helped!