1. **State the problem:** Solve the equation $$\frac{r}{4} + 9 = 21$$ for the variable $r$. 2. **Isolate the term with $r$:** To isolate $\frac{r}{4}$, subtract 9 from both sides: $$\frac{r}{4} + 9 - 9 = 21 - 9$$ $$\frac{r}{4} = 12$$ 3. **Solve for $r$:** Multiply both sides by 4 to get rid of the denominator: $$4 \times \frac{r}{4} = 12 \times 4$$ $$r = 48$$ 4. **Final answer:** The solution to the equation is $$r = 48$$.