An LCD projector in an office has a main light bulb with a reliability of $0\mathrm{.}9$ and a backup bulb, the reliability of which is $0\mathrm{.}75.$ The system is shown below. What is the reliability of the system?

$R_{\text{b}\text{a}\text{c}\text{k}\text{u}\text{p}}=0\mathrm{.}75$

$R_{\text{p}\text{r}\text{i}\text{m}\text{a}\text{r}\text{y}}=0\mathrm{.}9$