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The two electrical sub-network $N^{1}$ and $N^{2}$ are connected through three resistors as shown in the figure. The voltage across $5 \mathrm{ohm}$ resistor and $1 \mathrm{ohm}$ resistor are given to be $10 \mathrm{~V}$ and $5 \mathrm{~V}$, respectively. Then voltage across $15 \mathrm{ohm}$ resistor is

1. $-105 \mathrm{~V}$
2. $+105 \mathrm{~V}$
3. $-15 \mathrm{~V}$
4. $+15 \mathrm{~V}$