Let x be the length of the pen and y be the width of the pen.
The total cost of the pen is given by:
Cost = 3x + 5y = 300
3x + 5y = 300
3x = 300 - 5y
x = (300 - 5y)/3
The area of the pen is given by:
Area = xy = (300 - 5y)/3 * y
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