In the experimental set up of metre bridge shown in the figure, the null point is obtained at a distance of 40 cm from A. If a $10 \Omega$ resistor is connected in series with R1, the null point shifts by 10 cm . The resistance that should be connected in parallel with $\left(\mathrm{R}_1+10\right) \Omega$ such that the null point shifts back to its initial position is: