1. **State the problem:** Multiply 15 by 5. 2. **Formula used:** Multiplication of two numbers $a$ and $b$ is given by $a \times b$. 3. **Apply the formula:** Here, $a=15$ and $b=5$, so calculate $15 \times 5$. 4. **Intermediate work:** $$15 \times 5 = 75$$ 5. **Explanation:** Multiplying 15 by 5 means adding 15 five times or 5 fifteen times, which results in 75. 6. **Final answer:** $$\boxed{75}$$