題目如下：Norman International has a monopoly in the manufacture of whatsits. Each whatsit requires exactly one richet as an input and incurs other variable costs of $5 per unit. Richets are made by Rich Inc., which is also a monopoly. The variable costs of manufacturing richets are $5 per unit. Assume that the inverse demand for whatsits is Pw=50-Qw , where Pw is the price of whatsits in dollars per unit and Qw is the quantity of whatsits offered for sale by Norman International. Assume the two firms expect to last forever and that the discount factor R is 0.9. What is the maximum amount that Rich would be willing to pay the owners of Norman International to take over Norman ?(hint：calculate the present value of the profits of the two firms before and after the merge.) 我已經算出來合併前的利潤，Norman=100 Rich=200 合併後＝400 但discount factor那一段實在是怎麼樣都看不懂， 麻煩大家幫我解惑一下，非常感謝！！ --

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