A variety of pleasant items in the shop attract [plural] everybody.
A variety of animals are kept at the zoo.
A wide variety of dishes is essential for a successful restaurant.
A variety/range of products exists.
As far as I know "A variety" takes singular verbs.
Now my doubt is that in #1 and #2 it uses plural verb but in case of #3 and #4 it uses singular verb. Why so?
So does "A variety " takes singular or plural or both?
Somewhere on internet I read that we should treat "A variety" the same way we treat "A number". Is this true?
Some examples are also given:
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A number of people are causing trouble.
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The number of troublemakers is only about ten.
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A variety of books are on the table.
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The variety of books on the table is unbelievable.
I don't know if these are correct or not.
Best Answer
For any word or phrase that implies a set (e.g. variety, assortment, multitude, group, collection, lot, etc.), the best way to decide whether to treat it as singular or plural is to ask yourself "am I referring to the set's members, or to the set itself?" If you're referring to the set's members, use plural. If you're referring to the set itself, use singular (because, even though a set contains multiple things, it is a single thing).
Some examples might help to clarify this distinction. Imagine a florist conversing with one of her customers...
Example 1:
Note the use of the singular verb makes in the last sentence. Since the set is being treated as singular, we assume that the florist is making a statement about the set rather than its members. In other words, it's the variety, not the flowers, that she's identifying as that which makes a lovely bouquet.
Example 2:
The last sentence is exactly the same as the previous example's, except that the singular verb makes has been replaced with the plural make. The grammar is still correct, but the meaning has changed. This time, the florist is saying that any of the set's members can be used to create a lovely bouquet. She's not suggesting that the bouquet should contain the whole set (as in the previous example).
So both forms are correct, and the one you should use depends on the message you're trying to convey.
It's worth noting that many native English speakers-- myself included-- often mess this up, so you'll probably have to use context to figure out what we mean most of the time. Plural verbs seem to be preferred when the necessity of distinguishing between the set and its members is absent or unclear. In fact, you could probably get away with always using plural verbs. This is somewhat ironic since singular verbs have the advantage of not conflicting with the indefinite articles a and an (e.g. a variety, an assortment), which are exclusively singular. But for some reason, we prefer to break the rules in this case.
One reason may be the verb's close proximity to the noun that identifies the set's members (e.g. the flowers in "a variety of flowers"). Since that word is plural, it "feels right" to follow it with a plural verb, even if the actual subject of the sentence (the set) is singular.
Another reason may be that phrases like a variety of, a lot of, an assortment of, etc. are often used as substitutes for adjectives like various, many, some-- words that typically describe plural subjects and are therefor accompanied by plural verbs (e.g. "various flowers make lovely bouquets"). When people make this substitution, they may not feel comfortable changing the verb to its singular form because in their minds, they're still referring to multiple things as opposed to one.
So there is some gray area, but most of the time you'll do well to follow the rule described at the top of this post.
Note on definitions: The word variety has several meanings, of which the most relevant to the current topic is "diverse set". When it's preceded by the word the, as in "the variety of books on the table is unbelievable", it usually means "diversity" or "type". Those meanings have their own usage rules which are beyond the scope of this answer.