In the circuit shown in Fig, ${\mathrm{R}}_3$ is a variable resistance. As the value ${\mathrm{R}}_3$ is changed, current I through the cell varies as shown. Obviously, the variation is asymptotic, i.e., I ➔ 6A as ${\mathrm{R}}_3 \to \infty$. Resistances ${\mathrm{R}}_1 \mathrm{and} {\mathrm{R}}_2$ are, respectively :