Xiaohei designed the system above with her engineering team.

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

$$

$67\mathrm{.}73\%($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$$ system? $$$

$$

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

$$

enter your response here$\%($enter your response as a percentage rounded to two decimal$$ places$).$

$($d$)$ How much should they pay for R$5$ at the$$ maximum? $$$

$$

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