A's store monthly sales for April, May, June are three independent random variables with means and variances as follow:

April:
Mean: $9500
Variance: $1250

May:
Mean: $7400
Variance: $1425

June:
Mean: $8600
Variance: $1610

a) find the mean and standard deviation of the total sales for the 2nd quarter (3 month period).
-for mean do u just add up the three number and divide by 3 or you add up the three numbers and divide by the number of days in each month?

b) If the store manager is paid by a base monthly salary of $2000 plus 10% of the monthly sales as commission, what is the mean and standard deviation of his monthly salary for May?