How many light bulb jokes does it take to screw in a light bulb joke?
The probability that a given light bulb joke will be submitted to
the net in any given week is 0.4, and the probability that it will
have changed detectably since the last transmission is 0.2. Assuming
independence (which is reasonable, since no submitter of a light bulb
joke ever seems to know it has been submitted before within the last 2
or 3 weeks), the probability that it will change in a given week is
thus 0.08. So it takes about 12.5 light bulb jokes to change a
light bulb joke.
