1. **State the problem:** We have 100 students surveyed about their preference for Snapchat or Instagram. 2. **Given data:** - Total students: 100 - Boys surveyed: 50 - Boys who prefer Instagram: 40 - Girls surveyed: 50 - Girls who prefer Snapchat: 30 3. **Find:** Number of girls who prefer Instagram. 4. **Formula and reasoning:** Since all students have a preference, the number of girls who prefer Instagram is the total girls minus the girls who prefer Snapchat. 5. **Calculate:** $$\text{Girls who prefer Instagram} = 50 - 30 = 20$$ 6. **Answer:** There are **20 girls** who prefer Instagram.