I think it was exponential. Anyway, the problem gave three things. All three did the same thing and were turned on at the same time. The mean was some number, say 5. The idea is that you come back in 3 years and they want the probability that 1 thing is still doing the thing it was doing when it was turned on. It did not say exactly one thing or atleast one thing. It just said 1 thing is still working. So, how do I approach the problem? To me, if 1 thing is still working, then the other 2 are not. Either 1 stopped in year one, 2stopped in year 1, zero stopped in year one, one stopped in year two, or two stopped in year 2. But, this approach did not give me the right answer. I need guidance.

Thanks.

Thanks.

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