Answer
The mean for the combined sample is $9$.
Work Step by Step
Sample 1:
$M_1=\frac{(∑X)_1}{n_1}$
$7=\frac{(∑X)_1}{12}$
$(∑X)_1=12\times7=84$
Sample 2:
$M_2=\frac{(∑X)_1}{n_1}$
$12=\frac{(∑X)_2}{8}$
$(∑X)_2=12\times8=96$
Combined sample:
$M_C=\frac{(∑X)_C}{n_C}$
$M_C=\frac{(∑X)_1+(∑X)_2}{n_1+n_2}$
$M_C=\frac{84+96}{12+8}=\frac{180}{20}$
$M_C=9$
