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I found this sentence online and it had no reading to it. So I presumed that
“10” was read as **とお** considering how this number is counted within the
**つ** counter group.

**交わした10の約束は1つも守らなかった。**

However, I wasn’t sure so I asked a Japanese person whether it was really
**とお** or maybe **じゅう** and they said it was indeed **じゅう** , but they
couldn’t explain why. I was wondering if anyone knew? Is it perhaps that no
counter is necessary in this sentence? If the number in the sentence was 9 for
instance, would it be read as **きゅう** or **9つ** as in **ここのつ**?

Thanks in advance
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2020-05-16T15:58:31.723
77353
2020-05-17T12:08:21.707
29358
question
readings
Reading of the number 10 as とお or じゅう?
As you know

いち - ひとつ  
に - ふたつ  
さん - みっつ  
し - よっつ  
ご - いつつ  
ろく - むっつ  
なな - ななつ はち - やっつ  
きゅう - ここのつ  
じゅう - とお

There are corresponding words. And in your example, the latter one is already
decided ひとつ then the beauty of consistency, it should be とお.  
But sometimes とお is not recognized quickly and doubtless unlike じゅう, とお has
some homonyms and they are a little difficult to distinguish from another,
especially in speech.  
So the reason above, じゅうのやくそく sounds like a very common phrase, most people do
not mistaken. I prefer じゅう when I talk and I prefer とお when I read it together
(in a class or meeting).
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2020-05-16T23:13:33.790
77365
2020-05-16T23:13:33.790
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answer
じゅう and とお are On Yomi and Kun Yomi of the kanji 十

and it depends on where you are using it:

for saying 10 people or 10 things or 10 o'clock or 10th of the month, you use
じゅう

but for saying, for example, the 10th day: [十]{とお}[日]{か}
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2020-05-17T01:49:56.727
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2020-05-17T12:08:21.707
2020-05-17T12:08:21.707
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answer