Apple should charge $800 for it's laptop to deter dell from entering the market and earn 20% return on investment at this price.
Step-by-step explanation:
Apple will earn profit if it charges $1000 for it's very thin ad light laptop. If apple charges this much amount and dell does not enter the market, then apple would be able to earn highest profit. But if dell enters the market, then apple will not be able to earn highest profit and it will have to split the market with dell, earning about only 16%.
But if apple charges $800, then dell will not enter the market because it will be a loss for it because it would be only able to cover 5% of it's return on investment. Thus it is best for apple to charge $800 to deter dell from entering the market. At this price, apple will get 20% of it's return on investment.