[snip]

(2) Suppose that the amount of time that a lightbulb works before burning itself out is exponentially distributed with mean ten hours. Suppose that a person enters the room in which the lightbulb is burning. If the person wants to work for

hours, what is the probability that he will be able to complete his work without the bulb burning out? What if its not exponential.

Ok so I think

for exponentially distribution.

Mr F says: Correct. See also my main reply.
However, now I want to calculate

.

I would think that this would equal

.

Mr F says: Correct.