Internal Resistance Questions

Two sources of equal emf are connected to an external resistance $R$. The internal resistance of the two sources are $R_1$ and $R_2$ ($R_2>R_1$). If the potential difference across the source having internal resistance $R_2$ is zero, then:

In a potentiometer experiment the balancing with a cell is at length $240\text{ cm}$. On shunting the cell with a resistance of $2\Omega$ the balancing length becomes $120\text{ cm}$. The internal resistance of the cell is: