A population of 30 chemical processing units is to be sampled by a two-person crew. The mean time between calls for this operation is 68 minutes. If the mean service rate is one unit per minute, the cost of the crew and instruments will be $16 per minute. This cost will decrease to $12, $9, and $7 for service intervals of 2, 3, and 4 minutes, respectively. The time between calls for service and the service duration are distributed exponentially. If lost profit is $28 per hour for each unit that is idle, find the minimum cost service interval.