Let x be the common ratio and y be the 3rd term
11.25/x=y This means the fourth term divided by the common ratio will get the third term
20x=y This means the second term multiplied by the common ratio will get the third term
11.25/x=20x Set the two y's equal to each other, and solve for x!
20x^2=11.25
x^2=.5625
x=+-√.5625
x=.75 or -.75