$3.$ Determine the total cost for the following plan given the following forecasts: Month $123456$ Forecast $380400420440460480$ Use steady regular output of $400$ units per month, use overtime as needed for up to $40$ units per month, and use subcontracting to make up any shortage. Unit costs are: Regular output $=$ $$25$ Overtime $=$ $$40$ Subcontract $=$ $$60$ Inventory per month $=$ $$15$