I had an exam today (Egyptian GSEC english exam) and in a passage about library it said:

> 
> The book slipped from his hands and fell on the floor with loud noise, 20 eyes looked at him and his face turned red.....
> 
> 
> 

It then asked: approximately,how many persons where at the library?

Ten or twenty or forty

But I would like to know if twenty eyes represent 10 persons or is it like a metaphor and represents 20 persons?

Is this expression even correct?

Edit : in the passage it was obvious that it is a normal library with normal people i just cut a part out of its context that serves the question