Notice that the graph of the function is a parabola that opens downwards.
In general, a parabola that opens upwards/downwards is given by the expression below
[tex]\begin{gathered} (x-h)^2=4p(y-k) \\ p\rightarrow focal\text{ distance} \end{gathered}[/tex]
Thus, in our case,
[tex]\begin{gathered} -\frac{1}{20}x^2=y \\ \Rightarrow x^2=-20y \\ \end{gathered}[/tex]
After comparing the former result with the equation above,
[tex]\begin{gathered} \Rightarrow4p=-20 \\ \Rightarrow p=-5 \end{gathered}[/tex]
And the focal width is equal to |4p|; therefore,
[tex]\begin{gathered} \Rightarrow focal\text{ width}=|4p|=|4*-5|=|-20|=20 \\ \end{gathered}[/tex]
Therefore, the answer is focal width=20 units
