A class of 12 boys and 8 girls took a test. The girl's marks were 3, 6, 7, 8, 9, 9, 10 and 10.

1. Find the mean mark of the girls

There are $8$ girls.

Sum of girls' marks:
$3+6+7+8+9+9+10+10=62$

Mean mark:
$\frac{62}{8}=7.75$

1. Find the total marks for the boys and girls separately

Boys' total marks:
$12 \times 8.0 = 96$
Girls' total marks from part (a):
$8 \times 7.75 = 62$

2. Find the mean mark for the whole class

Total number of students:
$12+8=20$
Mean mark for the whole class:
$\frac{96+62}{20}=\frac{158}{20}=7.9$