In a cyclic quadrilateral ABCD, ∠DCB = 60°, the side length BC = 10 cm, CD = 30 cm and AD = 20 cm. DB is diagonal to the quadrilateral ABCD. If the area of triangle DAB is 60% of the area of the triangle DCB, then find the length of AB. (in cm)
1
20
2
9
3
3
4
15