Answer:
1.
The concept of opportunity cost is the benefit(s) which one has to sacrifice in order to pursue for other options/decision-making. In the described situation, the opportunity cost is the return on $20,000 investment on stock that earns 15% per year return as this investment has to be liquidated to finance for the house purchase.
2.
The fixed cost fallacy referring to the situation where economic decision-maker let irrelevant costs/ sunk cost(s) wrongly affect their decision-making.
The hidden cost fallacy is quite the contrary to the fixed cost fallacy; which refers to the situation where decision-makers fails to take into account relevant cost in their decision-making process.
3.
In the described situation, it is better to rent a house because it is more economical.
* If we rent the house, the total cost per year is $10,000
* If we buy the house, the total cost per year includes:
+ Cost of borrowing $80,000 + Opportunity cost of sacrificing 15% return per year on $20,000 stock investment = 80,000 *9% + 20,000*15% = $10,200;
So, it is cheaper to rent a house, rather than own it.
Step-by-step explanation: