Question 15.

$\text{Two linear equations } ax + by = c \text{ and } dx + ey = f \text{ have a unique solution if } \frac{a}{d}$ ≠ $\frac{b}{e}$

$\therefore \frac{k}{4}$ ≠ $\frac{1}{k} \Rightarrow k^{2}$ ≠ $4$

$\Rightarrow k$ ≠ $|2|$