In order to measure the internal resistance r _ 1 of a cell of emf...

In order to measure the internal resistance ${r}_{1}$ of a cell of emf $ε,$ a meter bridge of wire resistance ${R}_{0} = 50Ω$, a resistance $\dfrac{{R}_{0}}{2}$, another cell of emf $\dfrac{ε}{2}$ (internal resistance $r$) and a galvanometer $G$ are used in a circuit, as shown in the figure. If the null point is found at $l = 72cm,$ then the value of {r}_{1} = ____Ω .