The mean of a sample is:

$$\bar{x} = \frac{\sum (f_i \cdot x_i)}{N},$$

where $x_i$ is the class midpoint in hours per week, $f_i$ is the frequency, and $N = 200$ students.

$$x_1 = \frac{0+9}{2} = 4.5,$$ $$x_2 = \frac{10+19}{2} = 14.5,$$ $$x_3 = \frac{20+29}{2} = 24.5,$$ $$x_4 = \frac{30+39}{2} = 34.5.$$

$$20 \times 4.5 = 90,$$ $$50 \times 14.5 = 725,$$ $$60 \times 24.5 = 1470,$$ $$70 \times 34.5 = 2415.$$

$$\bar{x} = \frac{4700}{200} = 23.5.$$

The mean is $\boxed{23.5}$ hours per week.