now, we're assuming she made the deposit at the child's birth, namely when year was 0, at the child's twenty-first birthday that'll be 21 years later.

Answer:
10/18 and 20/36
Step-by-step explanation:
Factor x^4-y^4: (x²+y²)(x²-y²)
further factor: (x²+y²)(x+y)(x-y)
so the dimensions are (x²+y²)(length), (x+y) (width), and (x-y) (height)