A Resistor of Unknown Resistance and a 15- $\Omega$ Resistor Are Connected Across a 20-V Emf in Such a Are

Multiple Choice

A resistor of unknown resistance and a 15- $\Omega$ resistor are connected across a 20-V emf in such a way that a 2.0 A current is observed in the emf.What is the value of the unknown resistance?

A) 75 $\Omega$
B) 12 $\Omega$
C) 7.5 $\Omega$
D) 30 $\Omega$
E) 5.0 $\Omega$