a cool easy problem 4

We know that power $P= \dfrac {V^2}{R}$ , where $V$ is the voltage applied and $R$ the resistance of the conductor, in this case, the coil.

$\Rightarrow R = \dfrac {V^2}{P} = \dfrac {20^2} {800\times 4.2} = \dfrac {400}{3360} = \boxed{0.12} \space \Omega$