Answer
$21.7m$
Work Step by Step
First, we find the distance to Phillip as
$I_p=(\frac{R_B}{R_P})^2I_B$
This can be rearranged as:
$R_P=R_B\sqrt{\frac{I_B}{I_P}}=(12.5)\sqrt{2}=17.68m$
Now we can find the distance Brittany and Phillip as
$R_{BP}=\sqrt{(12.5)^2+(17.68)^2}=21.7m$
