Consider the right triangle in the attached diagram. Using the Pythagorean theorem, you can find the distance marked "a" by
r² = a² + o²
a = √(r² - o²) = √(20² -16²)
a = √(400 - 256) = √144 = 12
The maximum depth is the difference between r and a, so is (20-12) cm = 8 cm.
