Z=(127-120)/15=7/15 or 0.4667 approx. Z is simply the number of standard deviations a particular value is from the mean. This "normalises" the distribution so that only one table or graph is necessary to work out the probability corresponding to the value 127. (In this case the Z-score corresponds to about 68%, which means that 68% of the data is less than 127, so 32% is more than 127.)