At the mall every day, thousands of people pass through security on one of two levels: level C or level D. Suppose that, on average, it takes people 20 minutes to pass through security on level A with a standard deviation of 8.5 minutes to pass through security on level A. On level B, the mean and standard deviation are 18 and 6 minutes, respectively. Each day, the mall looks at separate random samples of 200 people from each level. They calculate the mean time for each sample, then look at the difference between the sample means (XA-XB) For boys, the average number of absences in the first grade is 15 with a standard deviation of 7; for girls, the average number of absences is 10 with a standard deviation of 6. Find the mean difference in the population and find the standard deviation of the difference. Sample for boys is 100 and girls is 50.