17.18 A property agent is investigating the average renting price per month for the factories in an area. In his study area, totally there are 8 types of factories. He randomly selected 4 different types of factories and stated down their average renting price per month and the numbers of factories as below.

Type of Factory Number of Factories Average Renting Price per Month

($000)

1 31 5 2 12 1.5 3 83 3 4 26 2.5

a. Estimate the average annual renting prices for the factories in that area.

b. Find a 90% confidence interval for the average renting prices for the factories in that area.