1. **State the problem:** We are given the ratio of girls to boys as 3:5 and the total number of students is 40. We need to find how many are girls. 2. **Formula and explanation:** The ratio 3:5 means for every 3 girls, there are 5 boys. The total parts in the ratio are $3 + 5 = 8$ parts. 3. **Calculate the value of one part:** Since the total number of students is 40, each part represents $\frac{40}{8} = 5$ students. 4. **Find the number of girls:** The number of girls is $3$ parts, so $3 \times 5 = 15$ girls. **Final answer:** There are 15 girls in the class.