Answer:
15 V
Explanation:
You want the voltage across parallel resistor R2 when the current through 30 Ω parallel resistor R1 is 0.5 A.
Ohm's Law
The voltage across resistor R1 is the product of its resistance and the current through it:
E = IR
E = (0.5A)(30Ω) = 15 V
Parallel circuits
All of the branches of a parallel circuit have the same voltage across them. The voltage across R2 is 15 V.