Given the Expenditure Side of Aggregate Output in the Economy

Given the Expenditure Side of Aggregate Output in the Economy:
§ Personal Consumption $200
§ Government Purchases 50
§ Gross Private Investment 40
§ Exports 30
§ Imports 40
a) Calculate the Gross Domestic Product
b) Calculate the Net Exports
c) Why only Final Sale Prices are included in the GDP?