how to calculate the probability of any statistic knowing X~N (statistical distributions)

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In my documentation they explain me how to calculate specific probabilities of some statistics

like $P(\bar X \ge x)$ or $P(S^2\ge x) $ knowing $X$~$N(\mu,\sigma)$

but then in the exercices they keep asking me things like this:

$P(S^2/\sigma^2 \le 2.041) $ with $ n=16$ and $X$~$(\mu,\sigma)$

or

$P(-1 \le [\bar X- \bar Y-(\mu_X-\mu_Y)]\le1)=95$% knowing: $\sigma^2_X=2$ and $\sigma^2_Y=2.5$ and $k=m=n$ (both are $N(\mu,\sigma)$ (find k)

How can I know the probability of a combination of statistics?( like those two examples) Is there any general rule to calculate a a combination of statistics for each distribution?

Thanks.