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I stumbled upon these 2 sentences:

这个城市人口比十年前增加了一倍

这个城市人口比十年前增加了两倍

The city’s population has doubled from a decade ago

I though always multiplies the number standing in front of it, it seems like in case of

How come? Is there any logical explanation?

  • 这个城市人口比十年前增加了一倍 = 这个城市人口是十年前的两倍, 这个城市人口比十年前增加了两倍 = 这个城市人口是十年前的三倍. – 賈可 Jacky May 26 '18 at 18:15
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    users suggest suspecting existence at this site of previous discussions of this matter, searching site seems to confirm such suspicion, see esp. chinese.stackexchange.com/questions/9295/… – user6065 May 26 '18 at 19:27
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Technically, when you say "增加了n倍", "n倍" is describing the difference the two number. So the original x becomes x*(n+1), while the difference is n*x.

If you want to say that the latter number is just n times the former, then it is "增加了(原来的)n倍"

However, in casual speech, people may not always respect this rule, which makes it confusing. Sometimes we have to make a confirmation to make sure we get the real number.

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They both express doubled.

It’s a bit like how:

  • a 100% increase (一倍)

is the same as

  • a two-fold increase (两倍)

A phenomenon that is equally confusing in English.


Compare: 一倍 example sentences from ABC

这座城市将要扩展一倍。 Zhè ⁴zuò chéngshì jiāngyào kuòzhǎn yị̄ ²bèi. This city will double its size.

学生人数增加了一倍。 Xuésheng rénshù zēngjiāle yị̄ ²bèi. The number of students has doubled.

and 两倍 example sentences from Tuttle

他的收人是三年前的两倍。 Tā de shōurén shì sān nián qián de liǎng bèi. His income is double what it was three years ago.

他现在挣的钱是先前工作的两倍。 Tā xiànzài zhèng de qián shì xiānqián gōngzuò de liǎng bèi. He's earning twice as much as in his previous job.

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  • Are you aware of the difference between 扩展/增加n倍 and X是Y的n倍? – dan May 26 '18 at 23:40
  • @dan Of course. Unless I misunderstood the question I believe that OP's issue is how 一倍 and 两倍 can mean the same thing. So I decided to use this angle to show how they both mean double. – Mo. May 28 '18 at 7:32
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Math:

倍 means multiple X * 1,

增加了一倍 means x + x * 1,

增加了2 倍 means x + x * 2

某某是它的2倍 means 某某 = x * 2

某某是它的1倍 means 某某 = x * 1, = x.

So, 我的分数是你的一倍 imply scores are equal technically.

The right one should be 我的分数比你多一倍.

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1

倍 is time in English. However, there are few expressions/phrases that can make difference when they are used in a sentence. I try to summarize a bit:

  1. X 比 Y 增加n倍: X = Y*(n+1);

  2. X 是 Y 的n倍: X = Y*n;

  3. X 比 Y 多n倍: X = Y*(n+1);

Those phrases are clear and will not cause any confusion.

In fact, the most confusing phrase is X 比 Y翻n倍. Some take it as X = Yn, some think X = Y(n+1), and some even think X = Y* 2^n. I indeed don't have any official answer for this so far.

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