The required maintenance workload for the next four months in a food processing plant is $550,400,360,$ and $445$ man$-$hours, respectively. The demand can be met by either regular time at a cost of SR $40$ per hour, overtime at a cost of SR $70$ per hour or subcontract at cost of SR $95$ per hour. Regular time capacities are listed following, overtime capacity cannot exceed $25\%$ of regular time capacity at any month. Late maintenance $($backlog$)$ costs extra SR $15$ per hour per Period. Subcontract hours are available but can only be purchased in $120-$hour contract. $($This means that you cannot use less than $100$ hours of subcontract hours, increment of $100$ hours$).$