This is a nice little problem.

$x(25 - x)(20 - 2x) = x(500 - 70x + 2x^2) = 2x^3 - 70x^2 + 500x \ne 2x^3 - 120x^2 + 500x.$

You got the conceptual part right and then messed up the algebra.

For part b, why would the dimensions be different for that part than for a?