Hi Cliff,

The circumference of a circle of radius r inches is 2 π r inches. Thus in your case

2 π r = 24 inches

so

r = 12/π inches.

The volume of a cylinder of radius r inches and height h inches (your pipe, standing on end) is

π r² h square inches.

Each cubic foot is 12 by 12 by 12 inches so a cubic foot is 12³ cubic inches. Thus for your pipe

 

π r² h = 2 × 12³ cubic inches.

But you know that r = 12/π inches so

π (12/π)² h = 2 × 12³.

Solving for h gives h = 24 π inches.

I hope this helps,
Penny