Trucks are required to pass through a weighing station so they can be checked for weight violations. Trucks arrive at the station at the rate of $18$ an hour between $7$:$00$ p$.$m$.$ and $9$:$00$ p$.$m$.$ Currently two inspectors are on duty during those hours, each of whom can inspect $15$ trucks an hour. Use Table $18\mathrm{.}4.$