1. **State the problem:** Susan has a round cake with a diameter of 20 cm. She wants to put a ribbon around the cake. We need to find the least length of ribbon required. 2. **Formula used:** The length of ribbon needed is the circumference of the cake, which is a circle. The circumference $C$ of a circle is given by the formula: $$C = \pi d$$ where $d$ is the diameter of the circle. 3. **Apply the formula:** Given $d = 20$ cm, substitute into the formula: $$C = \pi \times 20$$ 4. **Calculate the circumference:** $$C = 20\pi$$ 5. **Interpretation:** The least length of ribbon Susan can use is the circumference of the cake, which is $20\pi$ cm. 6. **Approximate value:** Using $\pi \approx 3.1416$, $$C \approx 20 \times 3.1416 = 62.832$$ So, the least length of ribbon needed is approximately 62.83 cm. **Final answer:** The least length of ribbon Susan can use is $20\pi$ cm, approximately 62.83 cm.