1. **State the problem:** Simplify the expression $15 + 9 \div 3$. 2. **Recall the order of operations:** Division and multiplication are performed before addition and subtraction. 3. **Apply division first:** Calculate $9 \div 3 = 3$. 4. **Perform addition:** Now add $15 + 3 = 18$. 5. **Final answer:** The simplified value of $15 + 9 \div 3$ is $18$.