Which is the correct one?
On average, the polyunsaturated fat content in one teaspoon of the olive oil and canola oil is the same
On average, the polyunsaturated fat content in one teaspoon of the olive oil and canola oil are the same
Which is the correct one?
On average, the polyunsaturated fat content in one teaspoon of the olive oil and canola oil is the same
On average, the polyunsaturated fat content in one teaspoon of the olive oil and canola oil are the same
On average, the polyunsaturated fat content in one teaspoon of olive oil and one teaspoon of canola oil is the same.
On average, the polyunsaturated fat content in two teaspoons of olive oil and two teaspoons of canola oil are the same.
The second one might sound more natural to the ear (a teaspoon is, two teaspoons are). However, I've incorrectly parsed the sentence. As @Santi pointed out in a comment, the teaspoons part of these sentences belong to the the prepositional phrase beginning with in, so we should really be keying on the subject of the sentence:
On average, the polyunsaturated fat content is the same.
That's true, and that's the right way to say it, no matter how many teaspoons (or tablespoons, cups, or milliliters) of oil we have:
On average, the polyunsaturated fat content in eight liters of olive oil and and eight liters of canola oil is the same.
If that sounds too awkward, then restructure the sentence such that there are fewer words between the subject and predicate.
is is specifically used for singular things (the subject of the sentence represents something that is singular--one thing) and are for plural things (the subject of the sentence represents something that is plural--many things). But be prepared to hear something like this from native speakers: "there's a lot of them" or "there's a bunch of cars in front of my house". That's just an example of daily English which oftentimes tends to disregard the prescriptive rules of standard English grammar.
First of all, I would completely rewrite your original sentence because they way you have it written right now sounds a little bit awkward, to say the least. I guess something like the following would do, though I have no idea what polyunsaturated fats are:
On average, the amount of polyunsaturated fats found in one teaspoon of olive oil is roughly equivalent to that found in one teaspoon of canola oil.
the amount is the grammatical subject of the sentence and it's one thing. Thus, we should use is.
On the other hand, if you still insist on using are, we can easily make the subject plural. Just like this:
On average, the amount of polyunsaturated fats in one teaspoon of olive oil and the amount of polyunsaturated fats in one teaspoon of canola oil are the same.
But that's rather a run-on sentence. I would highly recommend sticking with the first version.