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A resistor of unknown resistance and a $30-$ resistor are connected across a $20V$ emf in such a way that a $2\mathrm{.}0$A current is observed in the emf. What is the value of the unknown resistance?

Select one:

a$.30$OHM

b$.75$OHM

c$.15$ OHM

d$.7\mathrm{.}5$OHM