16×9

18×8

24×6

36×4

Let the larger radius be x. By the power of a point theorem,

AB BC = FB^{2}

x(x-42) = (x-24)^{2}

6x = 24^{2},

x = 96.

Now the smaller radius is AC/2 = (AB+BC)/2 = (2x -42)/2 = x-21 = 75. Hence the radius of the smaller circle = 75.