A colleague was wondering about a construction on the order of “Multiplying the number by 9 and adding the digits together give you a multiple of 9.” Does that sound odd to you? It did to her – she wanted it to be gives, not give. And yet the subject is two things: multiplying … and adding … So shouldn’t it take a plural conjugation, give?
It shouldn’t because it’s one action, multiplying and adding – a compound noun phrase that is nonetheless a single entity because it is a single complex action rather than two separate actions. If it’s two different possible actions – i.e., you can multiply or you can add with equal effect – then it’s plural. Parallel examples:
Kicking ass and taking names is my favourite Saturday evening pastime.
Kissing ass and taking bribes are both ways of getting ahead in business.
It’s similar to how we can say “The hop, step, and jump is the silliest track event,” not “are.”
When in doubt, though, or concerned that some readers may prefer singular while others prefer plural, you can always avoid the issue by using an auxiliary (or, as possible, a past tense), which conjugates the same either way:
Multiplying the number by 9 and adding the digits together will give you a multiple of 9.