妹は２１で大学生です。
tbd
My sister is twenty-one years old and a college student.
My sister is a twenty-one-year-old college student.
我妹妹21岁，是个大学生。
で
1. at, in
2. at, when
3. by, with
4. within, in
5. and then, so
6. and, then
7. let me tell you, don't you know
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