Please help with this question (cumulative distribution function of X)

[tiffyuyu]

The probability density function of the lifetime of a certain type of electronic device
(measured in hours), X, is given by

f(x) = 10/x^2,
0,
x > 10;
elsewhere.

(a) Find the cumulative distribution function of X, namely F(x) and hence find
P(X > 20).
(b) What is the probability that of 6 such devices, at least 3 will function for at least 20 hours? What assumption did you make?

 

[MarkFL]

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