So, between 18to10 and 54to30,which has a higher ratio?

By the Pythagorean Theorem:

d^2=x^2+y^2, where x and y are the dimensions of the rectangle, however this is just a square, so x=y=s so

d^2=s^2+s^2

d^2=2s^2, and we are told that s=6 so

d^2=2(6^2)

d^2=2(36)

d=√(2(36))

d=6√2  units

d≈8.49 units (to nearest hundredth of a unit)