P divides the segment joining A(-7, 2) and B(8, -8) in the ratio 3 : 2.
Find the sum of the coordinates of P.

$x=\frac{3\cdot 8+2\cdot (-7)}{5}=\frac{24-14}{5}=2$ and $y=\frac{3\cdot (-8)+2\cdot 2}{5}=\frac{-24+4}{5}=-4$, so $P$ = (2, -4) and the sum is $2+(-4)=-2$. The value -3 comes from swapping m and n, which gives the point (-1, -2); -2.5 sums the midpoint (0.5, -3); -5 simply adds the four given coordinates -7 + 2 + 8 - 8 without using the ratio at all.