The first 3 common multiples of 5 and 15 are 15, 30, and 45. Remember that the number itself is also a multiple of any number, since you can multiply it by 1.
Both mean and median are measures of the center of a set of data. Mean is normally preffered to the median except if there is an outlier that has a tendency to affect the mean. An outlier is a data point that significantly differs from the other data points in a sample. In the data set above, there is no outlier that will affect the mean. Therefore, he should use the mean because there are no outliers that affect the mean.
