[EX02-220] The following table shows the age distribution of heads of families:

Age of Head Age of Head of Family Number of Family Number 20–25 23 50–55 48 25–30 38 55–60 39 30–35 51 60–65 31 35–40 55 65–70 26 40–45 53 70–75 20 45–50 50 75–80 16

a. Find the mean age of the heads of families.

b. Find the standard deviation.