You can use a plural verb with a noun like “number" if it’s followed by a prepositional phrase with a plural noun, but you still need to be careful. (And if you're wondering why "M&M's" has an apostrophe, it's because that is the official product name.)
Rewrite Whenever Possible
But here’s the problem in my mind: a lot of people learned the “ignore the prepositional phrase” rule and have never heard of synesis. Scott and his editors even repeat the argument that they know the words “the proportion” should determine which verb they choose...and then they still write to me to see if I’ll give them permission or justification to do what they think is wrong. And that’s the problem: I’m giving you a justification, but a lot of people are still going to look at the sentence and think it is wrong. Or at least a few.
So if you embrace synesis, I don’t think you are safe—at least not safe from judgment. If you’re feeling bold, you can use a plural verb and know that at least some style guides are on your side and most people will think it sounds better; but if you don’t want to risk a few people thinking your sentence is wrong, it’s best to rewrite it.
Scott said they couldn’t think of a good rewrite, and I know it’s hard sometimes. Here are a couple or rewrites my editor and I came up with:
The original sentence was “We want to know the proportion of all M&M’s that are blue.”
My rewrite was, “Some proportion of all M&M’s will be blue. We want to know that proportion.”
My editor’s rewrite was, “Of all the M&M’s, we want to know the proportion that is blue.”
So that’s you Quick and Dirty Tip: You can use a plural verb with a noun of multitude like “handful,” “minority,” and “majority” if it’s followed by a prepositional phrase with a plural noun, but I still think there’s sometimes a reason to be cautious about doing so.
Example Sentences Using Synesis
TRADITIONALLY CORRECT: A number of girls is coming to the game.
CORRECT USING SYNESIS: A number of girls are coming to the game.
REWRITE: Many girls are coming to the game.
TRADITIONALLY CORRECT: A handful of countries has the death penalty.
CORRECT USING SYNESIS: A handful of countries have the death penalty.
REWRITE: The death penalty still exists in a handful of countries.
Mignon Fogarty is Grammar Girl and the founder of Quick and Dirty Tips. Check out her New York Times best-seller, “Grammar Girl’s Quick and Dirty Tips for Better Writing.”