1. **State the problem:** We need to find the area of a rectangular animal pen with a total fencing length of 200 meters, where one side is 60 meters. 2. **Formula used:** The perimeter $P$ of a rectangle is given by $$P = 2(l + w)$$ where $l$ is the length and $w$ is the width. 3. **Given:** $P = 200$ meters, $l = 60$ meters. 4. **Find the width $w$:** Substitute the known values into the perimeter formula: $$200 = 2(60 + w)$$ 5. **Solve for $w$:** $$200 = 120 + 2w$$ $$200 - 120 = 2w$$ $$80 = 2w$$ $$w = \frac{80}{2} = 40$$ meters. 6. **Calculate the area $A$:** The area of a rectangle is $$A = l \times w$$ $$A = 60 \times 40 = 2400$$ square meters. 7. **Answer:** The area of the pen is 2400 square meters.