Answer
The new mean:
$M=10$
Work Step by Step
$M=\frac{∑X}{N}$
There are 10 scores whose mean is $9$:
$9=\frac{X_1+X_2+X_3+X_4+X_5+X_6+X_7+X_8+X_9+X_{10}}{10}$
$X_1+X_2+X_3+X_4+X_5+X_6+X_7+X_8+X_9+X_{10}=9\times10=90$
Now, remove a person with a score of $0$. Since the position of this score in the sum above makes no difference, let's name this score as $X_{10}$
$X_1+X_2+X_3+X_4+X_5+X_6+X_7+X_8+X_9+X_{10}=90$
$X_1+X_2+X_3+X_4+X_5+X_6+X_7+X_8+X_9+0=90$
$X_1+X_2+X_3+X_4+X_5+X_6+X_7+X_8+X_9=90$
Find the new mean:
$M=\frac{∑X}{N}=\frac{X_1+X_2+X_3+X_4+X_5+X_6+X_7+X_8+X_9}{9}=\frac{90}{9}=10$
