Standard error of a sample

I want to calculate a standard error of a sample. I have a distribution of the IQ of the general population with a mean of 100 and a standard deviation of 19. Now I take a random sample of 30 students of Harvard and find that their mean IQ is 125.

I want to calculate the standard error of the mean. How can I do this? I cannot use $$\sigma \over \sqrt{n}$$ because I don't have the sd of my sample or?

• You need the sample standard deviation $\hat{\sigma}$ to estimate the standard error. – user2974951 Sep 26 '18 at 12:06
• Yes, thats what I thought. We don't have that. Is there another way like bootstrapping? – Jensxy Sep 26 '18 at 12:18
• You can only do bootstrap if you have a sample, which you don't, otherwise you could estimate the SD. – user2974951 Sep 26 '18 at 12:20
• I also know that the IQ is normal distributed. That's it. That are all information in the question. – Jensxy Sep 26 '18 at 14:53
• Then you are figuratively screwed. – user2974951 Sep 27 '18 at 6:02