I'd setup an equation, fill in the parts we know to deduce the SD since we know that a z-score is calculated by subtracting a data point from the mean and dividing by the standard deviation. We don't need to know the mean in this case, just that some score is 8 points above the mean.

So 8/s = .05 or I prefer to write it as .05s = 8 and solve for s which gives us the standard deviation. So divide both sides by .05 = a population standard deviation of 160.