The Central Limit Theorem - ~ N(0,1)

Where x is the observed value (in your case 23.5), x(bar) is the sample mean, and s is the sample standard deviation.

You want is P(Z<23.5), but first you need to standardize it using the CLT I mentioned above to get:

and then you would just use the normal table to obtain your percentage of how many student's sample mean is less than 23.5 (that's how I interpreted what the question was asking)