The Olive Garden Italian Restaurant records the amount of time customers wait for a table on Saturday evenings.

The following table lists the frequency of wait times for the last 100 customers.

Number of Minutes per Customer              Frequency

0 to under 10 …………………………………           6

10 to under 20 ……………………………….           22

20 to under 30 ……………………………….           10

30 to under 40 ……………………………….           18

40 to under 50 ……………………………….           28

50 to under 60 ……………………………….           16

a. Calculate the approximate average wait time per customer.

b. Calculate the approximate variance and standard deviation of the wait time per customer.