Help with Statistics homework!
What was the age distribution of prehistoric Native Americans? Extensive anthropological studies in the southwestern United States gave the following information about a prehistoric extended family group of 91 members on what is now a Native American reservation. For this community, estimate the mean age expressed in years, the sample variance, and the sample standard deviation. For the class 31 and over, use 35.5 as the class midpoint. (Round your answers to one decimal place.)
Age range (years) 1-10, 11-20, 21-30, 31+
# of individuals in above groups: 36, 25, 20, 10.
I got the mean as 15.9, but I need help with the last 2 parts.