Say you buy D deluxe models and B basic models.

You want to sell at least 100 microwaves, so

Your profit is at least 8000 so

These simultaneous equations give .

So in order to satisfy both conditions, you must sell at least 20 deluxe models and 80 basic models. But since you are asked to minimize the cost, we shall assume you are not to sell any more than that. So you should sell 20 deluxe models and 80 basic models.