A researcher conducted research with a large sample of drug users and found that the mean number of times they use illegal drugs each month is 12.34 with a standard deviation of 4.21.

(a)How should the researcher interpret the standard deviation value? In other words, what does the standard deviation value tell the researcher about drug use in this sample?

(b) What is the probability that a drug user in this sample would use drugs at least 15 times per month?

(c)What is the probability that a drug user in this sample would use drugs between 9 and 13 times per month?