I looked up a few grammar-teaching websites (in Chinese language). The website says many people do not use the phrase "A xxx of" correctly because they do not identify the subject of the phrase correctly.
The website says phrases like "A range of; A high percentage of" is singular, while "a number of; a group of; a couple of; a total of" is plural.
Therefore according to the website, the correct sentences should be as follows.
A wide range of courses is available. ("A wide range" is the subject)
A high percentage of peptides that are made of amino acids is present in the sample. ("A high percentage" is the subject)
A number/ group/ couple of people were killed in the earthquake.("people" is the subject )
A total of 230 employees were laid off. ("employees" is the subject)
I don't get the pattern here. All of this looks (or should I use "look"?) arbitrary to me. In fact I thought this phrase is supposed to be arbitrary and whether the phrase is plural or singular should depend on the the person's own judgment.
How do I identify the correct subject in the "A xxx of" phrase?