Quantas Airways Ltd.15 must schedule its hundreds of reservation salesclerks around the clock to have at least rt on duty during each 1-hour period starting at (24-hour) clock hour t = 0,c, 23. A shift beginning at time t extends for 9 hours with 1 hour out for lunch in the fourth, fifth, or sixth hours of the shift. Shifts beginning at hour t cost the company ct per day, including wages and night-hour premiums. Formulate an LP model to compute a minimum total cost daily shift schedule using the decision variables 1t = 0,c, 23; i = t + 4,c, t + 62 xi ! number of clerks working a shift starting at hour t yi, t ! number of clerks working a shift starting at hour t who take lunch during hour i