In: Advanced Math
Engineering system of type k-out-of-n is operational if at least
k out of n components are operational. Otherwise, the system fails.
Suppose that a k-out-of-n system consists of n identical and
independent elements for which the lifetime has Weibull
distribution with parameters r and λ. More precisely, if T is a
lifetime of a component, P(T ≥ t) = e−λtr, t ≥ 0. Time t is in
units of months, and consequently, rate parameter λ is in units
(month)−1. Parameter r is dimensionless. Assume that n = 8,k = 4, r
= 3/2 and λ = 1/10. (a) Find the probability that a k-out-of-n
system is still operational when checked at time t = 3. (b) At the
check up at time t = 3 the system was found operational. What is
the probability that at that time exactly 5 components were
operational? Hint: For each component the probability of the system
working at time t is p = e−0.1 t3/2. The probability that a
k-out-of-n system is operational corresponds to the tail
probability of binomial distribution: IP(X ≥ k), where X is the
number of components working. You can do exact binomial
calculations or use binocdf in Octave/MATLAB (or dbinom in R, or
scipy.stats.binom.cdf in Python when scipy is imported). Be careful
with ≤ and <, because of the discrete nature of binomial
distribution. Part (b) is straightforward Bayes formula.