The equation which represents the number sentence given in the task content is; n/3 + 9 = 21.
According to the task content, it follows that the sentence given is; Nine more than the quotient of a number and 3 is 21.
Since, the quotient of a number and 3 can be written as; n/3.
Consequently, the correct equation is; n/3 +9 = 21.
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