1. **State the problem:** Solve the proportion $\frac{2}{r} = \frac{12}{18}$. 2. **Formula and rule:** To solve proportions of the form $\frac{a}{b} = \frac{c}{d}$, use cross multiplication: $$a \times d = b \times c$$ This means multiply the numerator of the first fraction by the denominator of the second, and set it equal to the product of the denominator of the first fraction and the numerator of the second. 3. **Apply cross multiplication:** $$2 \times 18 = r \times 12$$ 4. **Simplify multiplication:** $$36 = 12r$$ 5. **Isolate $r$ by dividing both sides by 12:** $$\frac{36}{\cancel{12}} = \frac{12r}{\cancel{12}}$$ $$3 = r$$ 6. **Final answer:** $$r = 3$$ This method works for all proportions: cross multiply, then solve for the unknown variable by isolating it using inverse operations like division or multiplication.