A single$-$server queuing system has average time between arrivals of $20$ minutes and a service time of $10$ minutes each. Assuming Poisson arrivals and exponential service times, the utilization factor is approximately:

$0\mathrm{.}50.$

$2\mathrm{.}0.$

$0\mathrm{.}33.$

$0\mathrm{.}25.$