One revolution of a wheel is equal to it's circumference:
C = πd
C: circumference
d: diameter
20" wheel
d = 20"
C = (20π) in/rev
v = (24 rev/min)(20π in/rev) = 480π in/min
30" wheel
d = 24"
C = (30π) in/rev
v = (18 rev/min)(30π in/rev) = 540π in/min
[tex]Ratio = \frac{velocity of 30" wheel}{velocity of 20" wheel} = \frac{v_{2}}{v_{1}}[/tex]
[tex]\frac{v_{2}}{v_{1}} = \frac{540\pi}{480\pi} = \frac{9}{8}[/tex]