City bus drivers work two three and a half-hour shifts per day. In some cases, the two shifts are consecutive (effectively one seven-hour shift), but usually they are not. Because of the inconvenience of breaking up the day, those who work non-consecutive shifts are paid a $15 per day bonus. All bus drivers earn a base rate of $25 per hour. The bus company has the daily requirements in the table. Subject to meeting all its requirements for drivers, the bus company wishes to minimize its daily labour cost (regular and bonus)