其中 (among them / of which)

Thirty students, five from abroad

• Your teacher describes your new class to a visitor.
• we / us我们wǒ men班有bān yǒu三十个sān shí gèstudents学生xué sheng，of which其中qí zhōng五个wǔ gèis是shìstudent studying abroad留学生liú xué shēng。 — "There are thirty students in our class, five of whom are international students."

Group，of which其中qí zhōng + part

• of which其中qí zhōng = "among them, of which": it refers back to a group just mentioned and picks out part of it.
• It starts the second clause: I我wǒbought买mǎi(completed / change)了lethree三sānroot本běnbooks书shū，of which其中qí zhōngone一本yì běnis是shìa novel小说xiǎo shuō.

I我wǒbought买mǎi了lethree三本sān běnbooks书shū，of which其中qí zhōngone一本yì běnis是shìa novel小说xiǎo shuō。

The most, the best, a share

• these这些zhè xiēvegetable菜càiI我wǒall / both都dōulike喜欢xǐ huan，of which其中qí zhōngI我wǒmost最zuìlike喜欢xǐ huandumplings饺子jiǎo zi。 — "I like all these dishes; of them, I like dumplings best."
• With one of (sth)之一zhī yī: the Great Wall长城cháng chéngis是shìof which其中qí zhōngmost最zuìfamous有名yǒu míng(modifier / structural particle)的detourist attraction景点jǐng diǎnone of (sth)之一zhī yī。 — "The Great Wall is one of the most famous sights among them."

among them其中qí zhōngI我wǒmost最zuìlike喜欢xǐ huandumplings饺子jiǎo zi。

Name the group first

of which其中qí zhōng versus the middle中间zhōng jiān and li, ancient measure of…里lǐ

• of which其中qí zhōng refers to a group already named: of which其中qí zhōng两个liǎng gè.
• among在zài…(among)中zhōng / among在zài…li, ancient measure of…里lǐ names the group in the same phrase: among在zàithese这些zhè xiēstudents学生xué sheng(among)中zhōng，he他tātallest最高zuì gāo.

among在zàithese这些zhè xiēstudents学生xué sheng(among)中zhōng，he他tāmost最zuìtall高gāo。

Recap

Read the full reference: HSK 4 grammar — Nouns.