其中 (among them / of which)

Zoom into the group you just named

• among其中qí zhōng = "among them / of which" — points back into a group just mentioned.
• 三十个sān shí gèstudent学生xué shēng，among其中qí zhōng五个wǔ gèto be / is是shìschoolboy男生nán shēng = "thirty students, five of whom are boys."
• It zooms in on part of a set.

… ，among其中qí zhōng + (part)

• among其中qí zhōng refers to a subset of the group named before.

thirty三十sān shí(measure word)个gestudents学生xué sheng，among them其中qí zhōngfive五wǔ(measure word)个geare是shìboys男生nán shēng。

• among其中qí zhōng五个wǔ gèto be / is是shìschoolboy男生nán shēng = "five of them are boys."

Needs a group first

• among其中qí zhōng can't open a sentence cold — name the whole set first.

Recap

Read the full reference: HSK 4 grammar — Nouns.