r' = 4
Since all circles are similar to each other. Such as the radius always being half of the diameter. The ratio between the diameter and the circumference being pi, etc., we have plenty of information to calculate C, d, and r for both circles.
C/C' = 0.75 was given to us. So we know that d/d' is also 0.75. So let's write that equation and solve for d'
d/d' = 0.75
6.0/d' = 0.75
6.0 = 0.75d'
6.0/0.75 = d'
8 = d'
And r' will be half of d', so 8/2 = 4. Therefore r' = 4.
