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$).$