The focal distance of point (4, 2) on parabola ${\mathrm{x}}^2=8\mathrm{y}$ is
 

$$\begin{gathered} Given parabola {\mathrm{x}}^2=8\mathrm{y} \\ \Rightarrow 4\mathrm{a}=8 \\ \Rightarrow \mathrm{a}=2 \\ and given point \left({\mathrm{x}}_1,{\mathrm{y}}_1\right)=\left(4,2\right) \end{gathered}$$

 $$\begin{gathered} Now focal dis\tan ce =\left|{\mathrm{y}}_1+\mathrm{a}\right| \\ =\left|2+2\right| \\ =4 \end{gathered}$$