A sample with excess Ar40 and atmosphere Are given an apparent age that is younger or older than the real age? why?

Homework questions are not really allowed here, on this site.
Maybe someone will chime in, to say how you can go about finding the answer, but that’s about it.
No straight answers.

You don’t even tell us what it is you’re looking to find the age of.

This is a question about argon. Is Ar40 a stable isotope?

Oh! Thanks @stanleybmanly.

Ok. Turns out that this is the common form of argon in nature. So I guess the question is why a sample of atmosphere possessing an excess of argon would be dated younger or older than it is?

This is another of those questions that makes you wish you could get your fingers on the op’s throat. “Excess” compared to what? Fluctuations in the gas can vary for more reasons than you can count.

From the context, I take it that this is a question about potassium-argon (K-Ar) radiometric dating. If so, then errors in dating are almost always related to one of two things: whether the age of the sample is appropriate for the type of dating you are using, and whether the method’s assumptions are accurate.

K-Ar dating is primarily applicable to objects over 100,000 years old (as compared to carbon dating, which is for objects less than 50,000 years old). If the sample isn’t an appropriate age, then K-Ar dating will not give you an accurate result. So that’s one possible source of error.

If the age of the sample isn’t the problem, then you have to think about the assumptions that K-Ar dating makes. The chemical assumptions (rate of decay, independence of the decay process) aren’t good candidates for investigation. So ask yourself whether the sample came from a closed system or whether it might have been contaminated.

Those are the most likely starting places I can recommend, but you can also use context clues to narrow your search. Specifically, what are you working on in class right now? Most homework is designed to reinforce the concepts that were covered in class on the day it was assigned. So if any of the things I mentioned were talked about in class, it’s a good bet that’s what you’re being tested on.