Fractal wrote:could anybody explain me the difference between

a number of mutations --> which requires PLURAL

a set of mutations --> which requires SINGULAR

I don't understand how to handle such issues!

thx

1) A number of is plural (pls refer MGMAT SC guide). It means 'several'

2) For your second question, a set of mutations is singular because a set in the sense taken as a whole i.e; mass noun. Hence, it is singular. But we need context to decide.

How about these expressions :

a pair of, the pair of, a percentage of, the percentage of, three fifth of , A small percentage of the workers , an overwhelming proportion of women ?

GMATMadeEasy wrote:How about these expressions :

a pair of, the pair of, a percentage of, the percentage of, three fifth of , A small percentage of the workers , an overwhelming proportion of women ?

Unfortunately many of those are contextual in real English. You could easily see three fifths of the pie has been eaten but three fifths of the class have good grades. It depends on whether you are emphasizing individuals or the whole; in the case of the pie, there are no discrete individual elements of a whole, whereas people are arguably individuals.

Pair works similarly, though it will always be singular with pair of pants.

As an aside, "a number of" is never used when the number is 1, unless the speaker/author is intentionally trying to deceive.