Posted by Anonymous on Sunday, December 4, 2011 at 3:53pm.
Even numbers are 2, 4, 6, 8, 10, 12, 14, 16, 18, 20.
P(even) = 10/21
Multiples of 3 are 3, 6, 9, 12, 15, 18, 21.
Probability of either/or = sum of individual probabilities - common probabilities.