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A professor has recorded exam grades for $30$ students in his class, $1$ of the $30$ grades is unreadable. The mean score on the exam was $82$, and the mean score of the $29$ available scores is $84$,

What is the value of the unreadable score?

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    possible duplicate of [How the sample mean changes when you add a new observation](http://math.stackexchange.com/questions/198469/how-the-sample-mean-changes-when-you-add-a-new-observation)2012-10-21
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    Note that while one of the questions is expressed in terms of adding a new value and the other in terms of a value going missing, in both cases the mean of $n+1$ values is to be calculated based on the $(n+1)$-th value and the mean of $n$ values.2012-10-21

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