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I am not sure about the first part of

> 「中３の２５％、短文も理解困難」

Do you read 中 as ちゅう here? 25% of 3 would be 0.75% if I am not mistaken.
Therefore does it mean something like 「around 0.75% of the population does
have trouble with reading and understanding short sentences」? But then...
wouldn't one just write 0.75% in the beginning?
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2018-11-30T16:19:08.890
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2018-12-01T07:23:13.013
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question
meaning readings numbers
What does 中３の２５％　exactly mean?
yes, you read 中 as ちゅう , and 中３ as ちゅうさん

中３ is a shorthand way of writing 中学校3年生 which is **(Japanese) Junior High
School 3rd graders** or in other words, 9th graders.  
so 中３の２５％ would be "25% of (Japanese) 9th graders"

so the full translation of

> 「中３の２５％、短文も理解困難」

is

"Twenty-five percent of 9th graders have difficulty understanding short
passages."

Thanks to @Mars and @Eiríkr Útlendi for suggesting the more accurate word
choices.

After all this editing, I realized one final point that can be made: in the
expression 短文 **も** 理解困難, **も** carries the meaning "even", so the most apt
translation would be:

"Twenty-five percent of 9th graders have difficulty understanding even short
passages."
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2018-11-30T16:38:18.987
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2018-12-01T07:23:13.013
2018-12-01T07:23:13.013
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answer