In my assignment I am asked to determine a 90% confidence interval estimate of the mean “Sold Price” of the population of properties.

I don't really understand what that means? Is there a formula or something because I've been going at it for hours and I can't get it.

Mean is the average so it's probably the average of the sold prices.

Hooe this helped

First, you need to calculate the margin of error:

E = (z of alpha/2) * alpha / sqrt(n)

where alpha is 90% for your question and n is the sample size. Use a z-table to look up the z value for alpha/2

Your confidence interval is:

(x-bar - E) < mu < (x-bar + E)

where x-bar is your sample mean.