1. The problem asks us to predict the cost of the electricity bill after 15 days using the trend line equation. 2. The given trend line equation is $$y = 3x + 5$$ where $x$ is the number of days and $y$ is the cost of the electricity bill. 3. To find the cost after 15 days, substitute $x = 15$ into the equation: $$y = 3(15) + 5$$ 4. Calculate the value: $$y = 45 + 5 = 50$$ 5. Therefore, the predicted cost of the electricity bill after 15 days is 50. This method uses the linear equation to estimate values beyond the given data range by plugging in the desired $x$ value.