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I've found this sentence in a dictionary (LDOCE5), so I assume it is correct. But I can't understand why the noun salary is singular although it has a plural form. Could you explain it to me?
Research indicates that over 81% of teachers are dissatisfied with their salary.
As you pointed out there are several similar questions in this topic ( “On their back” or “on their backs”?, “Those who qualify will be awarded a certificate” or “those who qualify will be awarded certificates”?). I found Robusto's answer the most fitting that claims that teachers should have salaries in my sentence. But this answer implies that Robusto's answer is just a rule of thumb. So I can't find the explicit answer in the referred questions, I can only guess.
Is there a general rule that clarifies my sentence and the quoted answers?