Q: My son is completing his college application. In describing his efforts to teach rudimentary math to children at a community center, he’s written “three hammers plus one hammer equal four hammers.” Is it “equal” or “equals”? I think he’s right, but I’m not certain.
A: Either one is OK, though the singular usage (“equals”) is far more popular nowadays.
A couple of Google searches produced these results: “three plus one equals four,” 12,500 hits; “three plus one equal four,” only 7.
The choice of a singular or plural verb in such equations depends on whether you consider the first part a single unit or a compound.
Here’s what Garner’s Modern American Usage (3rd ed.) has to say on the subject:
“It’s possible to treat one and one as a single mathematical idea, so the appropriate verb is is. Or it’s possible to treat the two ones separately—hence are.”
Garner’s goes on to say that the same is true for multiplication: “both four times four is sixteen and four times four are sixteen are correct. But the singular is much more common and natural in modern usage.”
The Oxford English Dictionary’s entry for “plus” in this mathematical sense doesn’t get into the issue of singular versus plural verbs.
But the OED’s entry for the conjunction “and” says two numbers connected by it are “freq. treated as a unitary subject with singular verb.”
In fact, the earliest published reference in the OED for “and” used to connect two numbers (from a 1697 essay by Jeremy Collier) treats the subject as a singular: “The … notion … is as clear as that Two and Two makes four.”
However, the OED also has citations for the plural usage. Here’s one from Thackeray’s The Book of Snobs (1848): “When will you acknowledge that two and two make four, and call a pikestaff a pikestaff?”
In short, your son could use either “equal” or “equals” in his college application, but the singular is more popular now and would probably raise fewer eyebrows in the admissions office. We’d recommend going with it.
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