4. A closed rectangular box with sides x, y and height z is made with two kind of material. The top and the bottom cost $10 per square foot and the sides cost $5 per square foot. Find the construction...

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Extracted text: 4. A closed rectangular box with sides x, y and height z is made with two kind of material. The top and the bottom cost $10 per square foot and the sides cost $5 per square foot. Find the construction cost in terms of x and y given its volume is 16 ft'. Find dimension of the box so its cost of material is minimum.