The ratio, in which YZ-plane divides the line segment joining the points ( 4, 8, 10) and ( 6, 10, - 8), is 

Let  YZ-plane divides the line segment joining A (4, 8, 10) and  B(6, 10,-8) at P(x, y, z) in the ratio  k: 1. Then, the coordinates of P are

         $\left(\frac{4+6\mathrm{k}}{\mathrm{k}+1},\frac{8+10\mathrm{k}}{\mathrm{k}+1},\frac{10-8\mathrm{k}}{\mathrm{k}+1}\right)$

Since, Plies on the YZ-plane, its x-coordinates is zero

$\text{ i.e. }\frac{4+6k}{k+1}=0\text{ or }k=-\frac{2}{3}$

Therefore, YZ-plane divides AB externally in the ratio 2:3