Answer
The new mean is $8$
Work Step by Step
$M=\frac{∑X}{N}$
There are 7 scores whose mean is $9$:
$9=\frac{X_1+X_2+X_3+X_4+X_5+X_6+X_7}{7}$
$X_1+X_2+X_3+X_4+X_5+X_6+X_7=9\times7=63$
Now, add the new score: $X_8=1$
$M=\frac{∑X}{N}=\frac{X_1+X_2+X_3+X_4+X_5+X_6+X_7+X_8}{8}$
$M=\frac{63+1}{8}=\frac{64}{8}=8$
