thirty

三十的、三十个

Thirty people signed up for the beginners' cooking class.

有三十个人报名了初级烹饪课。

三十

Thirty is the number he always picks in the lottery.

三十是他买彩券时固定会选的号码。

三十岁、三十几岁常用复数

She went back to school in her early thirties.

她三十出头时重返校园读书。

三○年代常用复数

The film is set in the thirties, just before the war.

这部电影的背景是三○年代，就在战争爆发前。