The SET Follow Up

My friend John figured out the odds of a projector situation happening. Here is what genius John said...
"So I looked up the bulb life of overhead projectors and they seem to be between 25 and 75 ours depending on the type. Seems really low to me, 25 hours isn't even a whole week if you use the projector a lot, but that's what the retailers claim. Since the 75 hours are cheaper than 3-25hour bulbs I'll assume your school uses the longer life ones.Also assuming the projectors were on for < 10s (otherwise everyone probably could have read a short objective statement) and that those 10 s were the end of the 75 hours then the probability of one bulb burning out is:10s / (75hours * 60minutes/hour * 60s/minute) = 1 / 27,000. It happened twice so (1 / 27,000)^2.Then it was on one of the two SET observations per year. The average school year according to ask.yahoo.com is 180 days so now the probability is:(1 / 27,000)^2 * 2/180 or ~ 1/65,610,000,000.
Of course the above analysis assumes that the two burnouts are completely independent events which is very likely not the case. It could be that the bulb are replaced on a certain schedule and therefore bulbs in adjacent classes might very well be of similar usage and therefore both close to burnout. Or perhaps the problem was not the bulbs but a voltage spike in the outlet used, etc.Also this is ignoring the fact that turning bulbs on and off is much more stressful than leaving them on, but I couldn't find any info on the average number of ignitions before burnout."

Well... that is complicated but thanks John A.
But the bright side of having a SET that didn't go as planned... I came home to a beautiful boquet of daisies from my wonderful husband. Thank you Allen. I love you!