Willow Brook National Bank operates a drive$-$up teller window that allows customers to complete bank transactions without getting out of their cars. On weekday mornings, arrivals to the drive$-$up teller window occur at random, with an arrival rate of $18$ customers per hour or $0\mathrm{.}3$ customers per minute. What is the mean or expected number of customers that will arrive in a ten$-$minute period?