Find the standard deviation of the data summarized in the given frequency table.

The manager of a bank recorded the amount of time each customer spent waiting in line during peak business hours one Monday. The frequency table below summarizes the results. Find the standard deviation. Round your answer to one decimal place.

Waiting Time Number of Customers

0 - 3 11

4 - 7 15

8 - 11 13

12 - 15 10

16 - 19 0

20 - 23 1