Final answer:
The expected value for buying the Index Fund in the decision tree would depend on the potential returns and risks associated with the fund. It would be approximately equal to the average return of the stock market.
Step-by-step explanation:
The expected value for buying the Index Fund in the decision tree would depend on the potential returns and risks associated with the fund. Since the question states that buying any fund does not alter the trajectory of the economy, it implies that the performance of the index fund would closely mimic the overall behavior of the stock market.
Thus, the expected value for buying the Index Fund can be considered as the average return of the stock market over a given period of time. This expected return is calculated by considering historical data on stock market performance and analyzing the market trends.
For example, if the average return of the stock market is 8% per year, then the expected value for buying the Index Fund would be 8%.