[译] 60年代中期该校注册人数约为2,500人，包括约300名留学生。
[误] Enrollment of the university in the mid-1960 approximated 2, 500, including about 300 foreign students.

[正] Enrollment of the university in the mid-1960's approximated 2, 500, including about 300 foreign students.
[注] 凡表示年代时，在该年份之后加s或's，又如: The application of new technology began in the early 1990's. 90年代早期这项新技术开始应用。