约 + Number + Measure Word + Noun
约 (yuē) means "approximately" or "about" and is placed before a number to indicate an estimate. It is more formal than 大约 and is frequently used in news reports and written Chinese.
这座桥全长约两千米。
zhè zuò qiáo quán cháng yuē liǎng qiān mǐ.
This bridge has a total length of about two thousand meters.
会议约有三百人参加。
huì yì yuē yǒu sān bǎi rén cān jiā.
About three hundred people attended the meeting.
这件文物的历史约可追溯到一千年前。
zhè jiàn wén wù de lì shǐ yuē kě zhuī sù dào yī qiān nián qián.
The history of this artifact can be traced back to about a thousand years ago.