Your question is Variance of Sample Mean. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Prove the variance of the sample mean / explain why the standard deviation of the sample mean is sigma/sqrt(n).
Asked in the ML depth/breadth stage. Explain the assumptions required for the result, including independence and identical variance. Connect the derivation to experiment sample-size planning without introducing a business scenario.