I somehow associated using n-1 with computing the statistic for sample. Apparently is not the case with finding sample mean. If so, what is the difference between formulae for “sample mean” and “population mean”?

Population mean is (Total Population / N) Sample Mean is (Sample/n) The sample is chosen from the population because you believe it’s a good representation of the population. For exam purposes, you will be given the numbers. The n-1 your referring to is different. You use that denominator when calculating the Covariance when given historical data. The removes the statistical bias. Use that denominator when you see a question in portfolio mgmt asking you to calculate Covariance Covarance in Quant section uses the probability formula, which is slightly different. Hope this helps