A RAID$-0$ storage system has $4$ harddisks with failure rates of $\frac{0\mathrm{.}01}{?}$ day, $\frac{0\mathrm{.}02}{?}$ day, $\frac{0\mathrm{.}03}{?}$ day, and $\frac{0\mathrm{.}04}{?}$ day.

$($a$)$ What is the failure rate of the system $($per day$)?$

$($b$)$ What is the MTTF of the system $($in days$)?$