2.28 A drive-through fast-food chain obtained the following pick-up times (number of minutes between the time an order is placed and the time the order is picked up at the drive-through window). A random sample of 30 orders along with the corresponding duration is given in the following table:

Number of Minutes, xi Number of Orders, fi 2 6 4 11 4 6 6 8 6 6 8 6 8 6 10 5

a. What is the approximate mean pick-up time?

b. What is the approximate variance and standard deviation?