急求解一个英语问题！a half⼀half a

Jack,Helen,and（ half a dozen others） went on holiday.
为什么不是a half dozen others或a half dozen of others或half a dozen of others?

解答，half an hour (半小时） half a dozen others, 属于固定搭配，记住即可！

/'dʌzn; `dʌzn/
n (pl ~s or unchanged when counting sth 作量词时复数不变) (abbr 缩写 doz)
1 set of twelve （一）打（十二个）
*Eggs are sold by the dozen. 鸡蛋按打卖.
* They're 70p a dozen. 这卖70便士一打.
* Pack them in dozens. 按打装袋吧.
* [attrib 作定语] Half a dozen (ie 6) eggs, please. 我要半打鸡蛋.
* We need three dozen boxes. 我们需要三打盒子. =>App
4 见附录4.
2 (idm 习语) a baker's dozen => baker. one's daily dozen => daily. a dime a dozen => dime. dozens of (infml 口) lots of 许多
*She's got dozens of boy-friends. 她的男朋友多极了. talk, etc, nineteen to the `dozen talk, etc continually 不停地谈、说等
*They were chatting away nineteen to the dozen. 他们聊个没完. (it is) ,six of `one and ,half a dozen of the `other there is very little difference between the one and the other 两者几无区别; 半斤八
*I can't tell whether he or she is to blame it's six of one and half a dozen of the other. 我说不好他们俩谁是谁非--一个半斤, 一个八　.

number plural dozen or dozens
written abbreviation doz.
[Date: 1200-1300; Language: Old French; Origin: dozeine, from doze 'twelve']
twelve
a dozen eggs
two/three/four etc dozen
(=24, 36, 48 etc)
The number of deaths has risen to more than two dozen.
dozens of people/companies/cars etc
(=but not hundreds or thousands)
According to a government spokesman, dozens of people were killed.
Chris, Helen, and half a dozen others went on holiday together.
A dozen or so (=about 12) cars were parked near the entrance.
informal a lot of
a dozen
I've heard this story a dozen times before.
dozens of sth
She's had dozens of boyfriends.
We collected dozens and dozens of shells on the beach.
→ baker's dozen,a dime a dozen at dime , nineteen to the dozen at nineteen , six of one and half a dozen of another at six

1.At Least Half a Dozen Killed at Gaza Rally
2.Three killed, a dozen injured in knife attack at Belgian nursery

Except for today, prices range from $6 for a dozen short-stems to $19 for a dozen of its longest stems.
除非是当天的玫瑰，否则玫瑰的价格在短茎每打6美元和最长茎每打19美元之间。

2. Even on Valentine's Day, when the price of a dozen roses and delivery can soar as high as $150, 12 of Roses Only's most expensive flowers sell for just $35.
即使是在情人节，当一打玫瑰价格外加送花费可上涨至150美元的时候，"惟一玫瑰"花店里最贵的玫瑰也只卖35美元一打。

3. Every summer about a dozen journalists gather at a former army training camp north of London to spend the day watching the training of London's special armed police unit.
每年夏天，总有大约十几名记者聚集在伦敦北部的一个旧军训营，用一整天来观看伦敦特警部队的训练。

4. They are talking nineteen to the dozen.
他们哇啦哇啦地说个不停。

5. The lifeboat rescued a dozen people and took them to the larger ship.
救生艇救了十几个人，把他们送到大船上去。

6. The camp itself consisted of little more than hammocks strung between trees, the fires to keep off the jaguars, and half a dozen cooking pots.
他们的营地设备只有拴在树间的吊床，防范老虎的篝火和五六只炊锅。

7. These trails will link a dozen neighborhoods and the downtown business areas.
这些小道将把十几个居民区和市中心的商业区连接起来。

8. They put half a dozen Gestapo seals on my suitcases. I tried not to thank them too much.
他们在我的箱子上盖了6个盖世太保的图章。我尽量使自己不对他们的感谢过了头。

9. Nothing says love like a dozen long-stemmed roses on Valentine's Day.
没有什么比情人节的一束长茎玫瑰更能表达爱情。

10. Acting on a sudden impulse, I collected several dozen, put them in a paper bag, and took them to Robert.
我一时冲动，逮了几十只，装进一只纸袋里，带着去找罗伯特。
11.You wonder if eating a dozen chocolate-chip (碎片) cookies, wearing a house coat until 4, combing your hair at 5, cleaning behind the stove (twice) and crying in a job-agency parking lot qualify as symptoms of stress or maybe loss of self-worth.
你不知道这些情况是否可以归类于因紧张、或许还有失去自身价值所引起的症状：一连吃下十几块巧克力曲奇、下午4点钟还穿着居家便服、5点钟才梳头发、在炉子后面东扫西抹 (两遍），以及在职业介绍所的停车场上哭泣。

12. In fact, more than half of all roses are sold in groups of fewer than a dozen, says the Floral Index, a private firm that researches the flower industry.
据一家名为"花卉指数"的研究花卉业的私人公司指出，事实上，一半以上的玫瑰都不是成打出售的。

当名词前有half修饰的时候，冠词要放在half之后。
所以没有a half的说法，只有“half a ...”

另外，dozen的用法是：
a dozen (of) sth.（复数）
一般情况下of可以省略，但如果sth.前有限定词的时候则of不可省略。

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这是英美式英语的一大区别，英式英语喜欢用half a dozen, half a thousand, half an hour之类，而美式英语就是a half dozen, a half hour.大多数人应该更喜欢英式英语吧。去掉语言习惯的界限，你这四种说法均可成立。

固定用法，记住即可