17.21 A city is divided into 50 geographic subdivisions. An estimate was required of the proportion of households in the city interested in a new lawn-care service. A random sample of three subdivisions contained 611, 521, and 734 households, respectively. The numbers expressing interest in the service were 128, 131, and 172, respectively. Find a 90% confidence interval for the proportion of all households in this city interested in the lawn-care service.