Let the centre of the circle be O.
Here is how the diagram should look like:
AO = CO = 10 cm
Since D is opposite B, BD is also a diameter of the circle, meaning:
BO = DO = 10 cm
We know that triangle ABC is a right-angled triangle. We can find out BC using Pythagoras theorem.
AC² = AB² + BC²
20² = 12² + BC²
400 – 144 = BC²
BC² = 256
BC = 16 cm
We know triangle DBC is also a right-angle triangle with the hypotenuse being AC.
sine∠BDC = Opposite = BC = 16
Hypotenuse BD 20
16 ÷ 20 = 0.8