1. **State the problem:** In a class of 36 students, 19 read biology, 16 read chemistry, and 5 students do not read either subject. Find how many students read both biology and chemistry. 2. **Analyze the problem:** Total students = 36 Students not reading either subject = 5 So, students reading at least one subject = 36 - 5 = 31 3. **Use the principle of inclusion-exclusion:** Let $B$ be the set of students reading biology and $C$ be the set reading chemistry. We have $|B| = 19$, $|C| = 16$, and $|B \cup C| = 31$. By the inclusion-exclusion principle: $$|B \cup C| = |B| + |C| - |B \cap C|$$ 4. **Solve for the number of students reading both subjects:** $$31 = 19 + 16 - |B \cap C|$$ $$31 = 35 - |B \cap C|$$ $$|B \cap C| = 35 - 31 = 4$$ 5. **Final answer:** 4 students read both biology and chemistry.