1. **State the problem:** We need to find the length of the missing side $a$ in a right triangle where the hypotenuse is 20 and one leg is 16. 2. **Formula used:** In a right triangle, the Pythagorean theorem applies: $$a^2 + b^2 = c^2$$ where $c$ is the hypotenuse, and $a$, $b$ are the legs. 3. **Apply the formula:** Here, $b = 16$ and $c = 20$, so: $$a^2 + 16^2 = 20^2$$ 4. **Calculate squares:** $$a^2 + 256 = 400$$ 5. **Isolate $a^2$:** $$a^2 = 400 - 256 = 144$$ 6. **Find $a$ by taking the square root:** $$a = \sqrt{144} = 12$$ 7. **Conclusion:** The length of the missing side $a$ is 12.