Xiaohei designed the system above with her engineering team.

$($a$)$ The overall system reliability is

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d$)$ How much should they pay fo$67\mathrm{.}73\%($enter your response as a percentage rounded to two decimal $$places$).$

$($b$)$ If they have to pay $$$$3$ comma $500$ to fix a failed$$ system, what is the expected failure cost of the$$ system? $$$

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enter your response here $($round your response to the nearest $$dollar$).$

$($c$)$ If Xiaohei installs R$5$ as a backup for R$4.$ R$5$ has a reliability of$95\%.$ The probability of failure of the system with R$5$ is

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$($r R$5$ at the$$ maximum? $$$

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