Given that D is the midpoint of AC, and AC = 30 cm, then AD = DC = 15 cm
A right triangle is formed, where BD and DC are the legs and BC is the hypotenuse. Using Pythagorean theorem:
BC² = BD² + DC²
25² = BD² + 15²
625 - 225 = BD²
√400 = BD
20 cm = BD