In Wheatstone bridge, four resistors of resistances $15 \mathrm{\Omega }$, $12 \mathrm{\Omega }$, $4\mathrm{\Omega }$ and $10 \mathrm{\Omega }$ respectively, are connected in cyclic order. The resistance (in $\mathrm{\Omega }$) that is to be connected in parallel with the resistance of $10 \mathrm{\Omega }$ to balance the network is