17.22 A bank holds 720 delinquent mortgages in residential properties. It required an estimate of the mean current appraised value of these properties. Initially, a random sample of 20 was appraised, and a sample standard deviation of $37,600 was found. If the bank requires a 90% confidence interval for the population mean extending $5,000 on each side of the sample mean, how many more properties must be appraised?