Solving Isotopic Symbols Problems: Ar-40 & Beyond

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Meseria
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I am in an online chemistry course and its going well except for this particular type of problems. It states letters and tells me to write what isotope it is. Here goes...

Write isotopic symbols of the form X-A (e.g., C-13) for each of the following isotopes.

the a isotope with n(subscript a) neutrons. That's all it says. Nothing else. So i guessed trying like A-a but that was not it. I clicked show answer and it says it is Ar-40. How does one arrive at that? How does a isotope with n(sub a) neutrons = Ar-40?

Then it goes b isotope with n(sub b) neutrons, c with n(sub c), and d with n(sub d). I would like to know how this works. It feels like i am playing riddles.

There are other similar questions after this group but i think if somebody explains how you go from a isotope with n(sub a) neutrons to Ar-40 i will be able to figure out the rest for myself.

I could post an image of the questions if this doesn't make any sense.

Thank you.
 
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I think someone forgot to insert the actual numbers into the problem. As it stands, the answer does not follow from the given information (unless a and na are defined somewhere).
 
I am posting a screen shot of the problem. So does this make any sense to anybody?
 

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Meseria said:
I am posting a screen shot of the problem. So does this make any sense to anybody?

Yeah dude, I saw the picture. and I am doing mastering chemistry too as I googled explanations on why N sub a was an actual element. but apparently when I typed "Ar-40" mine was incorrect, for my (a) answer was "Ar-36". So, I guess by random, the isotopes are switched from time to time. and we're just going to have to forfeit the points. luckily, my instructor grades upon completion rather than percentage of correct answers, so hopefully yours does too.

So, I believed you asked, how they came to that conclusion. And my only conclusion is that there was a typo error on the website where somebody making the homework problems just forget to edit the "x" values (a & a sub n, etc...) before it was completed. That's my only guess.
 
When I first looked at this, I thought it would be something like A-(p + n (sub a)), but for that, you would need to know the number of protons (p) in A. I'm guessing someone forgot to give the values when inputting the problem, like everyone else said.