1. **Problem statement:** Suppose the ratio of the ages of two siblings is 3:5. If the sum of their ages is 40 years, find their individual ages. 2. **Formula and rules:** Let the ages be $3x$ and $5x$ respectively, where $x$ is a common multiplier. 3. **Set up the equation:** Since their sum is 40, we write: $$3x + 5x = 40$$ 4. **Simplify the equation:** $$8x = 40$$ 5. **Solve for $x$:** $$x = \frac{40}{8}$$ 6. **Calculate $x$:** $$x = 5$$ 7. **Find individual ages:** - First sibling's age: $3x = 3 \times 5 = 15$ - Second sibling's age: $5x = 5 \times 5 = 25$ 8. **Answer:** The siblings are 15 years and 25 years old respectively.