The lifetime in hours of a certain kind of radio tube is a random variable having probability density function
$f(x) = \begin{cases} \frac{100}{x^2} , & x>100 \\ 0, &otherwise\\ \end{cases}$
What is the probability that exactly 2 of 5 such tubes in a radio set will have to be replaced within the first 150 hours of operation?
MY ATTEMPT:
To be quite honest with you I'm having trouble understanding this question
But I tried to use:
$ p(X>100; n=0.4) = \int_{100}^{150} f(x) * (0.4) $
I'm pretty sure this is wrong but I'm not entirely sure of another way that this can be done.
This is not homework. I'm studying past-exams for an exam I have on Tuesday.
Some help would be great.