Outliers are no problem

For points $(x_i,y_i),$ the best-fit line $y=mx+b$ is given by the formulas $\begin{aligned} m &= \frac{n \left( \sum x_iy_i \right) - \left( \sum x_i \right) \left( \sum y_i \right)}{n \sum x_i^2 - \left( \sum x_i \right)^2} \\ b &= \frac{\left( \sum x_i^2 \right) \left( \sum y_i \right) - \left( \sum x_i \right) \left( \sum x_iy_i \right)}{n \sum x_i^2 - \left(\sum x_i\right)^2} \end{aligned}$

In our case, this gives $m = \dfrac{10 (400) - 55(40)}{10 \cdot 385-55^2} = 24/11$ and $b=\dfrac{385(40)-55(400)}{10 \cdot 385-55^2} = -8,$ so the answer is $24+11-8 = \fbox{27}.$