Let x be the value of the third term.
Since it's a geometric sequence, then:
Third term/Second term = Fourth term/Third term
x/20 = 11.25/x
Cross multiplication:
x^2 = 225
x = 15
So since the ratio is constant, then it is
x/20 = 15/20 = 3/4
So the common ratio is 3/4 or 0.75.
