A piece of string $72$ cm long is to be used to tie up a small "rectangular" parcel. The string must pass twice around the width of the parcel and once around the length. What is the largest volume ($cm^3$) that the parcel can have? Disregard the insignificant amount of string used to tie the knot.
This is a question in a grade $7$ and $8$ math contest. Obviously, there is no calculus involved. How can we solve this?