A walk in clinic for emergency room services maintains records of the number of patients it treats per day. The following table shows the frequency of the patient arrivals over the course of a 150 day period:

Number of Patients per Day            Frequency

20 to under 40 ………………………..          10

40 to under 60 …………………………        16

60 to under 80 ………………………….       25

80 to under 100 ……………………….         65

100 to under 120 ………………………        34

a. Calculate the approximate average number of patients per day.

b. Calculate the approximate variance and standard deviation of the number of patients per day.