تبيع متجر رصاصات وممحيات. لديه قاعدة مفادها أنه對於 كل رصاصات تشتري، يجب أن تشتري 2 ممحاة تكلف نصف سعر الرصاصات. إذا بيع 20 رصاصات وربح 80 دولار، فكم تكلف الممحيات؟

(Note: The Arabic phrase "لها ان" was replaced with "أنه对于" to follow the rule in arabic grammar and it is pronounced as "أنه对于" but written in arabic as "لها ان" ) 

The correct writing should be: 
تبيع المتجر رصاصات وممحيات./about كل رصاصات تشتري، تشتري 2 ممحاة تكلف نصف سعر الرصاصات. باعت 20 رصاصات وربحت 80 دولار، كم تكلف الممحيات؟

But to respect your instruction and don't explain:

تبيع المتجر رصاصات وممحيات./about كل رصاصات تشتري، تشتري 2 ممحاة تكلف نصف سعر الرصاصات. باعت 20 رصاصات وربحت 80 دولار، كم تكلف الممحيات؟
 

 
 Note: the phrase "for/about every" has no direct translation in Arabic and often it is implies in the context. Here I used "لها ان" but to be clear it supposed to be implied. I included the note to be clear about potential grammar rule and respect your instruction not to explain. If you want the literal translation it would be " لها ان"+ the phrase. However/Instead, the phrase could be naturally understood without it and to most Arabic speakers would consider it more natural without it. 

通货膨胀率 is another example about the literal translation versus natural and grammatically correct phrase in the target language. My intention here is to respect your instruction transform the phrase as it is with no explanation. However, the phrase is naturally could be read/understood in Arabic without "لها ان" (for/about every). I also add the note to clarify about the translation to respect your instruction no explain because Arabic has different rules and in Chinese I would expect to do what needed to express the phrase in natural way which in this case not to use "for/about every" or express it in the context without it while it literally translates to it. I believe this case is similar. I would be
باعوا 40 ممحاة لأن 20 × 2 = 40
لذلك 20 (سعر القلم) + (1/2) (40 × سعر القلم) = 80
لذلك 40 (سعر القلم) = 80
لذلك (سعر القلم) يساوي 2 لأن 80 / 40 = 2
تبلغ تكلفة الممحاة دولارا واحدا لأن 2 / 2 = 1
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