Answer
$1.67Km$
Work Step by Step
We know that
$\frac{I_2}{I_1}=\frac{P/4\pi r_2^2}{P/4\pi r_1^2}$
This simplifies to:
$\frac{I_2}{I_1}=(\frac{r_1}{r_2})^2$
This can be rearrangned as:
$r_2=r_1\sqrt{\frac{I_1}{I_2}}$
We plug in the known values to obtain:
$r_2=(1.0)\sqrt{\frac{2.80\times 10^{-6}}{10^{-12}}}=1.67Km$
