Answer:
24 Ohms
Step-by-step explanation:
Resistance is a function of amperes. For each amp, there is one and only one value for resistance.
It works the other way, too.
E=IR (well-known relationship). If IR=12, R=12/I
R=E* (1/I)
When the current is 0.5 amp, R=12/0.5 - 24 ohms
