Answer
$34.64\ cm$
Work Step by Step
Step 1. Based on the given information, we have
$a=\frac{40}{2}=20, b=\frac{20}{2}=10$
Thus
$c=\sqrt {a^2-b^2}=10\sqrt 3$
Step 2. The electrode is at the other focus; thus the distance between the electrode and the kidney stone is
$2c=20\sqrt 3\approx34.64\ cm$
