还是米有想出来。。。

我是这样理解的：
1. 需求函数由于每购买一个椰子的价钱相当于以前的2倍所以 D=1200-200P
  因为需求是居民的，供给是国民的所以需求是供给的一半D=S/2
  解出来是S=480
2.由于市场的所有供给全部由居民消费所以D=1200-200P,D=S，解为400
虽然想是这么想的但还是觉得别扭。。。

找一下范里安的练习册答案：忘记文件名了，标题说是偶数答案，不过我印象是全部答案（下文是我早先复制出来的）。

16.11 (1) King Kanuta rules a small tropical island, Nutting Atoll,
whose primary crop is coconuts. If the price of coconuts is P, then King
Kanuta's subjects will demand D(P) = 1; 200 − 100P coconuts per week for their own use. The number of coconuts that will be supplied per week by the island's coconut growers is S(p) = 100P.

(a) The equilibrium price of coconuts will be 6 and the equilibrium quantity supplied will be 600.

(b) One day, King Kanuta decided to tax his subjects in order to collect coconuts for the Royal Larder. The king required that every subject who consumed a coconut would have to pay a coconut to the king as a tax. Thus, if a subject wanted 5 coconuts for himself, he would have to purchase 10 coconuts and give 5 to the king. When the price that is received by the sellers is pS, how much does it cost one of the king's subjects to get an extra coconut for himself? 2pS.

(c) When the price paid to suppliers is pS, how many coconuts will the king's subjects demand for their own consumption? (Hint: Express pD in terms of pS and substitute into the demand function.) Since pD = 2pS, they consume 1; 200 − 200pS.

(d) Since the king consumes a coconut for every coconut consumed by the subjects, the total amount demanded by the king and his subjects is twice the amount demanded by the subjects. Therefore, when the price received by suppliers is pS, the total number of coconuts demanded per week by Kanuta and his subjects is 2; 400 − 400pS.

(e) Solve for the equilibrium value of pS 24/5 , the equilibrium total number of coconuts produced 480 , and the equilibrium total number of coconuts consumed by Kanuta's subjects. 240.

(f) King Kanuta's subjects resented paying the extra coconuts to the king, and whispers of revolution spread through the palace. Worried by the hostile atmosphere, the king changed the coconut tax. Now, the shopkeepers who sold the coconuts would be responsible for paying the tax. For every coconut sold to a consumer, the shopkeeper would have to pay one coconut to the king. This plan resulted in 480=2 = 240 coconuts being sold to the consumers. The shopkeepers got 24=5 per coconut after paying their tax to the king, and the consumers paid a price of 48=5 per coconut.

恩，弄明白了是这个样子的~~~多谢啦

这里其实涉及到很多关键性概念，不妨拆解一下，那位做出了正确答案却感到别扭的朋友为什么会纠结。
首先，根据题意有三个价格：市场价格P1，居民价格P2，国王价格P3，已知p3=0,根据总支出和总收入恒等原则可以得到式p2+p3=2p2,这个式子的左边是总支出，右边是总收入。所以得p2=2p1。
第二，只有一个供给s，但是有两个需求，居民的需求d1,国王的需求d2,已知d1=d2,则总需求d=2d1。
根据总供给与总需求恒等原则，有s=d
将上述参数代入各式即得解。楼上已经完成，不赘叙。