Thank you for your answer, but I don't really get your notation.

I have the standard deviation of the sample, s = 200

The mean of the sample is at most 100 more than the mean of the population. I interpret that as = the standard error of the mean is 100.

The equation for the standard deviation for the mean is

...but it seems that I can't extract n from there (well, I can with a little algebra but the answer makes no sense). Why ? Beside, even if I could get 'n' from the equation, I would have no interval of confidence.