don't post the same solution given here in Chegg I will give dislike

I need the decision variables, constraints and objective function.

The demand for an item over the next four quarters is 300,400,450 and 250 units, respectively. The price per unit starts at $20 in the first quarter and increases by $3.00 each quarter thereafter. The supplier can provide no more than 400 units in any one quarter. Although, we can order more in early quarters and store them, however a cost of $3.80 is incurred per unit per quarter and you cannot store more than 80 units in any one quarter. Determine the model that constructs optimum schedule to purchase the item to meet the demand with minimal cost