The trend line of annual sales of a company is given as Y = 30 + 3.6 X Where Y = annual total sales and…

When $Y$ represents annual total sales and $X$ is measured in years, converting the dependent variable from annual sales to monthly sales divides the equation by 12, and changing the time unit $X$ from 1 year to 1 month divides the slope by 12 again. Thus, the monthly rate of increase (slope) is $\frac{3.6}{12 \times 12} = \frac{3.6}{144} = 0.025$.