Based on the data, the most likely correlation coefficient would be -1. The slope between 20 and 30 days is -1, and it represents the change in the surface area of the lake per day. The data represents correlation, not causation.
Since the data would form a perfectly straight line through the points, the correlation coefficient would be -1 for a perfect decreasing fit. To find the slope, find the change in the surface area between those days, the change in the days, and write it as surface area/days: 80-90=-10; 30-20=10; -10/10=-1 This is not causation because there could be lurking variables we cannot see.
