1. **State the problem:** In a class of 30 students, two-thirds of them are girls. We need to find how many girls are in the class. 2. **Formula and rule:** To find a fraction of a quantity, multiply the total quantity by the fraction. 3. **Calculate the number of girls:** $$\text{Number of girls} = \frac{2}{3} \times 30$$ 4. **Simplify the multiplication:** $$= \frac{2 \times 30}{3}$$ 5. **Cancel common factors:** $$= \frac{\cancel{2} \times 30}{\cancel{3}} = 2 \times 10 = 20$$ 6. **Interpretation:** There are 20 girls in the class. **Final answer:** 20 girls.