Because Core Writing and Core Math are so important for the rest of your education at the University, you will be required to take English and Math courses until you complete the core requirement.
For example: if you placed (based on your ACT/SAT scores) into ENG 101, you must take ENG 101 your first semester and then ENG 102 the next semester. ENG 102 completes the core writing requirement.
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The official, catalog statement on core English and math completion:
Competence in writing and mathematics is foundational to success at the university. Therefore, it is the policy of the University of Nevada, Reno that all degree-seeking students must be continuously enrolled in appropriate pre-Core or Core mathematics and English courses until the University Core Curriculum mathematics and English requirements are completed. For further information, contact your college advisement officer, the Advising Center, or the Office of Admissions and Records.