Index of refraction for a substance is
(speed of light in vacuum) divided by (speed of light in the substance) .
So . . .
(299,792,458 m/s) / (speed of light in Lucite) = 2.25
Speed of light in Lucite = (299,792,458 m/s) / 2.25
= 133,241,092 m/s
(That's 1/2.25 = 44.4% of the speed of light in vacuum.)
