1. Problem statement: Compute $15 \div 3$. 2. Formula and rules: Use $a \div b = \frac{a}{b}$ and require $b \neq 0$. 3. Important rule: If $a = b \times c$ then $\frac{a}{b} = c$ for $b \neq 0$. 4. Intermediate work: Express $15$ as $3 \times 5$. 5. Evaluation: Substitute to get $\frac{15}{3} = \frac{3 \times 5}{3} = 5$. 6. Final answer: $15 \div 3 = 5$.