The following grouped data show the frequencies of daily sales volume (in hundreds of dollars):

Daily Sales Number of Days
0.25 and under 1 3
1 and under 1.75 9
1.75 and under 2.5 10
2.5 and under 3.25 4
3.23 and under 4 2
4 and under 4.75 12
4.75 and under 5.5 6
5.5 and under 6.25 1
6.25 and under 7 3
7.0 and under 7.75 2

a. Calculate the mean daily sales.
b. Calculate the standard deviation of the mean daily sales.