The formula to calculate the z-score of a certain value is:
z = (x - (mean))/(standard deviation)
In this example, the x-value is 450, the S.D. is 100, and the mean is 500.
So, substitute these values into the formula.
z = (450-(500))/100
z=-50/100
z=-0.5
Therefore, the z-score is -0.5.