По оценкам оператора сотовой связи средняя длительность

According to the mobile operator the average duration of daily calls of 24 minutes per user. A sample survey of 100 customers showed that the average daily duration of the call is 30 minutes. At the level of α = 0,05 vote statistically significant difference sample survey, it is known that the standard deviation of duration of calls in the general population is 3 minutes.