Question

if the scale factor between two circles is 2x/5y, what is the ratio of their areas?

Answer 1

you need to know in reality what mean this scale factor of 2x/5y between these two circle

Answer 2

in general, the ratio of two areas is the square of the linear ratio

quick derivation for a circle:
let a = pi * r^2 be the smaller circle, and A = pi * R^2 be the larger circle

then A/a = (pi * R^2) / (pi * r^2) = R^2/r^2 = (R/r)^2