data has a mean of 150 and a standard deviation of 10. What percentage of the observed values fall between 140 and 160?
The empirical rule states that within one standard deviation of the mean is 65% percent of the data. Which is the case in this problem, it is one standard deviation below and one standard deviation above so 65% is the answer. You could do z scores where z = x bar - mu / sigma but the empirical rule is ideal for this problem.