Answer
The new mean:
$M=15$
Work Step by Step
$M=\frac{∑X}{N}$
There are 6 scores whose mean is $13$:
$13=\frac{X_1+X_2+X_3+X_4+X_5+X_6}{6}$
$X_1+X_2+X_3+X_4+X_5+X_6=13\times6=78$
Now, remove a person with a score of $3$. Since the position of this score in the sum above makes no difference, let's name this score as $X_6$
$X_1+X_2+X_3+X_4+X_5+X_6=78$
$X_1+X_2+X_3+X_4+X_5+3=78$
$X_1+X_2+X_3+X_4+X_5=75$
Find the new mean:
$M=\frac{∑X}{N}=\frac{X_1+X_2+X_3+X_4+X_5}{5}=\frac{75}{5}=15$
