Find k, if the line 4x - y = 1 is perpendicular to the line 5x - ky = 2?

Answer: Option B

$$\begin{array}{rl}& \text{Given,}\\ & 4x-y=1\\ & \therefore y=4x-1\\ & 5x-ky=2\\ & \therefore y=\frac{5x}{k}-\frac{2}{k}\\ & y=m_{1}x+C\\ & m_1=\frac{5}{k}\end{array}$$
If the lines are ⊥ then the product of their slope is -1
$$\begin{array}{l}{m}_1\times m_2=-1\\ 4\times \frac{5}{k}=-1\{\begin{array}{l}{m}_1=4\\ m_2=\frac{5}{k}\end{array}\\ \therefore k=-20\end{array}$$