The probability on Buffy being in Las Vegas is like you said 1 out of 40.
The odds on plaw being in Vegas is 1 out of 90.
The odds on both of you being in Vegas at the same time for any particular day is
1/40 * 1/90 = 1 / 3600, which seems really small.
However, you can correct it by knowing that people usually go to Vegas on weekends. Out of 52 weekends in a year, let's say 48 are Vegas eligible weekends (correcting for the holidays, etc.)
Therefore, Buffy is in Vegas 3 out of 48 weekends (= 1/16). Plaw is in Vegas 1 out of 48 weekends. The probability that Buffy and Plaw are together at the same time is:
1/16 * 1/48 = 1/768
Note that I'm doing the probabilities as separate independent events. It makes a difference if we know one of the parties is in Vegas to begin with.
