Xiaohei designed the system above with her engineering team.

$($a$)$ The overall system reliability is $\%($enter your response as a percentage rounded to two decimal places$).$

$($b$)$ If they have to pay $$3,600$ to fix a failed system, what is the expected failure cost of the system? $($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 $70$ The probability of failure of the system with R$5$ is $(/$enter your response as a percentage rounded to two decimal places$).$

$($d$)$ How much should they pay for R$5$ at the maximum? $($round your response to the nearest dollar$).$