In order to measure the internal resistance $$r_1$$ of a cell of emf 𝐸, a meter bridge of wire resistance $$R_0 = 50 Ω$$, a resistance $$R_0/2$$, another cell of emf E/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 = 72 cm, then the value of $$r_1 = ___ Ω$$.