In a survey to estimate average household monthly medical expenses, 500 households were selected at random from a population of 5000 households. Of the selected households, 336 had children in the household and 164 had no children. A strati?ed subsample of 112 households with children and 41 households without children was then selected, and monthly medical expenditure data were collected from households in the subsample. For the households with children, the sample mean expenditure was $280 with a sample standard deviation of 160; for the households without children, the respective ?gures were $110 and 60. Estimate mean monthly medical expenditure for households in the population, and estimate the variance of the estimate.