A line of best fit for a data set is y = 2.5x + 10. Using this model, what value of y is predicted when x = 8?

Substitute $x=8$ into the line of best fit: $y=2.5(8)+10=20+10=30$, choice H. The slope of $2.5$ means each increase of $1$ in $x$ adds $2.5$ to the predicted $y$, and $10$ is the predicted value at $x=0$. Choice F ($20$) leaves out the $+10$. Choice G ($28$) comes from an arithmetic slip, such as $2.5\cdot7.2$. Choice J ($40$) doubles the product, $2.5\cdot16$ or $20+20$. Make sure you multiply before you add. A prediction for a value of $x$ within the range of the data is more reliable than one far outside it.