To mean of a set is given by the sum of all values in the data-set divided by the number of values.
We have that the mean of the whole set is 82.
The mean of the 9 readable scores is 84.
So:
[tex]\begin{gathered} \frac{x}{9}=84 \\ x=84\cdot9 \\ x=756 \end{gathered}[/tex]So, we 9 readable scores add up to 801. If we add 756 to a number, y, and divide by 10, we'll have the mean score of the exam, 82.
[tex]\begin{gathered} \frac{756+y}{10}=82 \\ 756+y=820 \\ y=820-756 \\ y=64 \end{gathered}[/tex]So, the grade of the unreadable score was 64.
