1. **Problem statement:** Suppose the ratio of the ages of two siblings is 3:5. If the older sibling is 20 years old, find the age of the younger sibling. 2. **Formula and rules:** When ages are in ratio $a:b$, the actual ages can be represented as $3x$ and $5x$ for some $x$. 3. **Set up the equation:** Given the older sibling's age is 20, we have: $$5x = 20$$ 4. **Solve for $x$:** $$x = \frac{20}{5}$$ $$x = 4$$ 5. **Find the younger sibling's age:** $$3x = 3 \times 4 = 12$$ 6. **Answer:** The younger sibling is 12 years old. This method uses the concept of ratios to find unknown ages by scaling the ratio parts to actual values.