Let's call a the lenght of the square end, l the lenght and g the girth

Volume = a^2*l or 4a+l=90 so l=90-4a

Volume = a^2(90-4a)=90a^2-4a^3

We want to maximize the volume

V'(a)=180a-12a^2=12a(15-a)

V'(a)=0 so a=0 or a=15

The only possible correct value is a=15 inch and l=30 inch

Please best me if satisfied. Thanks

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