核心问题
（1）Joe asked for fifty cent's worth of chocolate candy. (《英语惯用法大词典》，中国科技出版社，p. 2054)
(2) We put three dollar's worth of gasoline in the tank. (同上)
请问 three dollar's worth 与 three dollars' worth 哪个用法是正确的？
词典解答
可能有人认为，名词作定语应取单数形式。但是，请记住，time 和 worth 之前的名词定语，如具有复数意义，则应采用复数形式并加上所有格形式'或's，或者广而言之，当用名词的属格（所有格）作定语时，则具有复数意义的名词应采用复数形式：
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They stole fifty thousand dollars' worth of equipment. (EED) 他们偷了价值五万美元的设备。
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He bought ten dollars' worth of postage stamps. (London) 他买了十美元的邮票。
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She sold fifty dollars' worth of tickets. (Chambers) 她卖了50美元的票。
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They had got away with thousands of pounds' worth of diamonds. 他们带着价值数千英镑的钻石逃之夭夭了。
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He needs a few days' rest. (Nelson) 他需要休息几天。
(参见 in three minutes)