Assume that the following are data from cluster sampling with simple random sampling of clusters. There are 10 clusters (primary units) and a total of 100 secondary units in the population. For each of the n = 3 selected clusters, yi is the cluster total for the variable of interest and Mi is cluster size: y1 = 4, M1 = 5; y2 = 12, M2 = 20; y3 = 7, M3 = 10. (a) Give an unbiased estimate of the population total. (b) Estimate the variance of that estimator.