Here's the last question.... (I had two other threads...)

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Question:

Even though they are in refrigerated storage, apples spoil at a rate of 20 kg/mo. The September price for apples is $2.50/kg and increases $0.25/kg each month. Storage costs are $150/mo. and 1000 kg of apples are placed in storage in September. When should the apples be sold for maximum profit?

I have no idea where to start