Power is current times voltage. Using Ohm's Law V=IR, we can express power in two additional ways:
[tex]P=I^2R \\ P= \frac{V^2}{R} [/tex]
So for P=75W, we can use either of these to get R. Let's use the first one:
[tex]75W=(0.5A)^2R \\ \frac{75W}{0.25A^2}=300 \Omega [/tex]
