A bottling company uses a machine to fill plastic bottles with pop. Due to variations in the filling process, the pop amounts in all bottles are normally distributed with a mean of 300 ml and a standard deviation of 3 ml. One bottle is randomly selected, what is the probability that bottle contains less than 299.5 ml?

$\displaystyle z=\frac{299.5-300}{\frac{3}{\sqrt{n}}}$ I'm not sure what value n is, or if it's needed.

I think I'm suppose to take n=1.

For this question how I'm calculating the probability do I write $\displaystyle P(\bar{x}<299.5)$ or $\displaystyle P(x<299.5)$?