recall the "inscribed angle" theorem, an angle inscribed is half the intercepted arc
now, the line BA goes through the center, and to each end, thus is a diameter, the point of tangency, where a tangent hits the diameter or radius of a circle, is always a right-angle
The ratio of the smaller to the larger is given as 2:4. Another way to write this is as a fraction: which of course you can see now will reduce to . That's your ratio.
which of course you can see now will reduce to 
. That's your ratio.