My reasoning: (A(x,y))dz/dt = -8.8rad(z) should be the setup. I need the area of vertical elliptic cross-sections, each with area xy(pi) with x and y increasing as the fluid approaches the halfway point and decreasing as the rest of the container drains. Once I have the area function I can solve the separable DE. The issue is that I don't know where to begin in expressing the area as a function of z.

Thanks