Resistances are connected in a meter bridge circuit as shown in the figure. The balancing length $l_{1}$ is $40 \mathrm{~cm}$. Now an unknown resistance $x$ is connected in series with $\mathrm{P}$ and new balancing length is found to be $80 \mathrm{~cm}$ measured from the same end. Then the value of $x$ will be ____________ $\Omega$.