You are in charge of buying office supplies for your business.  Your workers use red and black pens.  Red pens cost $5 per box and black pens cost $3 per box.  You need at least 10 boxes of pens.  You want no more than 7 boxes of black pens and no more than 6 boxes of red pens.  You want to minimize the cost to your business.
The objective function for this situation is C = 5x + 3y.  What (x, y) pair minimizes cost?
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(6, 7)
(0, 7)
(3, 7)
(6, 4)