The manager of a bank recorded the amount of time each customer spent waiting in line during peak business hours one Monday. The frequency distribution below summarizes the results. Find the standard deviation of the frequency distribution. Round your answer to one more decimal place than is present in the original data values.

Waiting Time (minutes) / Number of Customers 0-3 / 5 4-7 / 7 8-11 / 1 12-15 / 7 16-19 / 5 20-23 / 10