TIP: Questions as such often become simpler instantly once we recognize certain lengths to be the radius of a circle. (Any line segment joining a circle from the circumference to its centre is its radius!) In this case, we can work with BC being a radius of the larger circle. The rest is manipulation and solving of equation.
TIP 2: you may easily substitute the answer back into equations (1) and (2) to CHECK if one is indeed 30% of the other!