From an article about surgery:
"The number and size of the incisions depends on the surgery and equipment needed."
There are two subjects here ("number" and "size")--a compound subject--but a singular verb ("depends"). Compound subjects contain an "and," so if you are connecting two items with an "and," you need a plural verb. It's easy to forget about the first part of the compound subject, so that's why this is such a common error.
Here's the rewrite:
"The number and size of the incisions depend on the surgery and equipment needed."
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