7. The lifetime in hours of a certain kind of radio tube is a random variable having a probability density function given by

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? Assume that the events Ei , i = 1, 2, 3, 4, 5, that the ith such tube will have to be replaced within this time are independent.

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f(x)=100 +2 x 100 x > 100