Current Electricity

In a potentiometer experiment, when two cells of emfs $E_1$ and $E_2\left(E_2>E_1\right)$ are connected in series, the balancing length is 160 cm . If one of the cells is reversed, the balancing length decreases by $75 \%$. If $E_1=1.2 \mathrm{~V}$, then $E_2=$