How to understand "Of " at the beganning of the sentence

Question

Of the students sitting in a room by themselves, 75 percent got up and found someone to alert about the smoke.

why use "of " in this sentence? can we omit "of"?

Answer 1

The of is required. Do not omit it.

Here is your sentence:

Of the students sitting in a room by themselves, 75 percent got up and found someone to alert about the smoke.

And here it is again written more conventionally:

75 percent of the students sitting in a room by themselves got up and found someone to alert about the smoke.

In this version, you should be able to see that of is required. We often write phrases like X% of something. Dividing the entire phrase into its parts and changing the sequence does not change the requirement.

The writer has probably shifted the prepositional phrase to the beginning of the sentence in order to make the grammatical subject of the independent clause (75 percent) easier to perceive at a glance.

Answer 2

It's an inverted form of "75 percent of the students sitting in the room by themselves got up and ...". Hence you need the of.

Omitting the "of" will leave you with a run-on sentence that is incorrect.