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.

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.

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Type of Factory 1 234 4 Number of Factories 31 12 83 26 Average Renting Price per Month ($000) 51 1.5 3 2.5