If a piece of equipment has a mean time between failures $($MTBF$)$ of $500$ hours with a mean time to repair $($MTTR$)$ of $10$ hours, then its uptime or average availability would beIf a piece of equipment has a mean time between failures $($MTBF$)$ of $500$ hours with a mean time to repair $($MTTR$)$ of $10$ hours, then its uptime or average availability would be