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Two slices of cake is ______ dollars.

Why is it is when two slices is plural?
Comments  
It is a single purchase.
Can you please explain with more detail?
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This is case where you should consider the "amount", and not the number of things.

Two and two is four. (= Two plus two equals four)
Ten dollars is not enough for us. (= That amount of money is not enough...)
Two slices of cakes is ten dollars. (= The price, the amount of money you need is ten dollars... or, the amount of stuff you are considering is worth ten dollars)

I think this use of the singular is especially common in certain structures, but it's not always fixed. For example, I think I would normally say:
Two slices of cake cost ten dollars.

Just my opinion.
What about:

How come two x-rays is so expensive? Or How come two x-rays are so expensive? I think the 2nd one sounds correct.

But two x-rays could be counted as a set or single purchase too, right?
pleasehelpBut two x-rays could be counted as a set or single purchase too, right?
It could be one purchase or two. What is in your head when you say it?

What makes some of these constructions so difficult is that we want rules that ignore the meaning inside people's heads. Go by "what's in your head". Emotion: smile

How come two X-rays are so expensive? (In your head: Why are those two purchases of those two X-rays so expensive?) (You are thinking of the two X-rays as two separate units.)

How come two X-rays is so expensive? (In your head: Why is that purchase of two X-rays so expensive?) (You are thinking of the two X-rays as a single unit.)

CJ
Students: We have free audio pronunciation exercises.
The
CalifJimHow come two X-rays are so expensive? (In your head: Why are those two purchases of those two X-rays so expensive?) (You are thinking of the two X-rays as two separate units.)

How come two X-rays is so expensive? (In your head: Why is that purchase of two X-rays so expensive?) (You are thinking of the two X-rays as a single unit.)

CJ

So two x-rays can count as a single thing or multiple things but if it were multiple that would be more then one action.
Thanks again!