1. **Problem statement:** We need to find how many students are selected in total for the competition if 20% of the boys and 25% of the girls from all classes are selected. 2. **Step 1: Calculate total boys and girls across all classes.** - Boys: $15 + 18 + 12 = 45$ - Girls: $20 + 22 + 18 = 60$ 3. **Step 2: Calculate the number of boys selected.** - 20% of boys means $0.20 \times 45 = 9$ 4. **Step 3: Calculate the number of girls selected.** - 25% of girls means $0.25 \times 60 = 15$ 5. **Step 4: Calculate total students selected.** - Total selected = boys selected + girls selected = $9 + 15 = 24$ 6. **Answer:** The total number of students selected is **24 students**. Therefore, the correct choice is **C 24 students**.