Let's find the pattern in the ratios given:

7/12 : 4/5
5/18 : 25/27
3/4 : 33/20
9/16 : 3/64

It seems like the numerator of the second ratio is the product of the two denominators in the first ratio, and the denominator of the second ratio is the sum of the two numerators in the first ratio.

Applying this pattern to the last example:

25/72 : 45/63

=> Numerator: 45 (9 x 5)
=> Denominator: 45 (25 + 20)

Therefore, the missing ratio would be 45/45.