大约 + Number/Quantity
大约 (dàyuē) is an adverb meaning 'approximately,' 'about,' or 'roughly.' It is placed before a number or quantity to indicate an estimate rather than an exact figure. It is interchangeable with 大概 (dàgài) in most contexts but slightly more precise in tone.
从这里到机场大约要四十分钟。
cóng zhèlǐ dào jīchǎng dàyuē yào sìshí fēnzhōng.
It takes about forty minutes from here to the airport.
这个班大约有三十个学生。
zhège bān dàyuē yǒu sānshí gè xuéshēng.
This class has about thirty students.
她大约下午三点到。
tā dàyuē xiàwǔ sān diǎn dào.
She'll arrive at about three in the afternoon.