Which is correct?:
A and B combined is sufficient
A and B combined are sufficient.
...Basically, is the proper interpretation of 'A and B' as two nouns or 'A and B combined' as a single noun?
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In terms of meaning, if you use the singular verb with "A and B combined," you are suggesting "A and B combined" should be viewed as a single entity, like "the set A and B." If you use the plural verb, you are suggesting "A and B" should be viewed as at least two things, A and B, which are combined.
In terms of usage, I did a search on COCA to see what I could turn up. I imposed a couple of commonsense restrictions: A and B both have to be singular elements (or I'd always expect are), and the noun phrases joined by a conjunction have to be the primary subject (so nothing like "sales of A and B combined are..." which relegates A and B to a prepositional phrase modifying sales).
I found three results for combined are:
Health promotion and health education combined are ... ("A Profile of Western (USA) Higher Education Physical Education Degree Programs," Physical Educator, Early Winter 2006)
Medicare and Medicaid combined are ... ("Eldering: Aging with Resilience," Futurist, Jan/Feb 2013)
Good nutrition and regular exercise combined are ... ("Exercise can minimize side effects of drugs used in cancer treatment," News-Medical.net, 4-30-2016)
Here are the results for combined is:
Government study finds Prozac and CBT combined is ... ("Patient, Fix Thyself" Saturday Evening Post, 2007 (Jul/Aug))
A pre- and pro-biotic combined is ... ("Make Way for Mycoprotein in U.S. Food Supply," Consumers Research Magazine, 2001 (Sep))
I can't detect a significant difference in frequency from that sample. The usage really seems to vary based on how the writer is thinking of A and B combined, and not based on a universal rule. Is may be more common in a caption or headline (as the excerpt from the Saturday Evening Post appears to be), or if it's clear the resulting item is a single thing ("a pre- and pro-biotic combined" is a "synbiotic," a single thing.
Both are correct. If A and B are regarded as one thing, they should be followed by ' is'. If A and B are regarded as two separate things, just use 'are'. Examples are from the iWeb Corpus https://www.english-corpora.org/iweb/:
1.Dinner and entertainment combined is $50.
2.Agriculture and tourism combined is what is better known as agritourism.
3.Once your engine's hooked up, it really helps to have the back of the car raised substantially because the engine and gearbox combined are very long and need a particular angle of attack to go in.
4." Sometimes, if the humidity is too high or the heat and humidity combined are high, it could affect what you are going to do the rest of the day, " he said.