Jack drives down to town every morning. On 16 randomly chosen trips, he recorded his travel time xi (i is a subscript!) in minutes.

He finds that $\displaystyle \displaystyle\sum_{i=1}^{16} xi = 519 $ and $\displaystyle \displaystyle\sum_{i=1}^{16} (xi)^2 = 16983 $

Calculate the unbiased estimates of the variance of the driving times to town.

I could calculate the unbiased estimate of the mean, but I dont get how to get the variance from$\displaystyle \displaystyle\sum_{i=1}^{16} (xi)^2 = 16983 $. What can I get from the sum of (xi)^2? I know I can easily get the mean from the sum of xi, but (xi)^2? Isn't the variance the sum of ((xi-mean)^2) / n so it has nothing to do with the sum of (xi)^2?

Thank you so much in advance, really appreciate this!