If we want the area to be more than 225 sqft, and the width is already established as 25, then we can divide 225 by 25 to see what the minimum would be. 225/25 is 9, so this is the minimum. We want a set of possible lengths though, and as long as the pen is over 225 sqft, it's allowed. Therefore, the answer is
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hope this helps! comment if it's confusing