A TV manufacturing company uses speakers at the rate of 8000/mo. When it places an order for speakers it incurs a fixed cost of $1200. The monthly interest rate for keeping a speaker in stock is assessed at 1%/mo. The cost of the speaker depends on the order size. If less than 1000 speakers are ordered the cost is $11 each. When the order size is between 1000 and 10,000 the cost is $10.50/unit. For order sizes between 10,000 and 30,000 the cost is $10 per unit. For order quantities between 30,000 and 80,000 the cost drops to $9.50. Beyond 80,000 the cost is $9.25. Determine the optimum order size and time between orders if shortages are not allowed.

Please use excel to solve this problem. Show all cell formulas and parameters used.