Hello,

the volume of the silo is:

. Rearrange to get

The surface area is:

The price is

. Now plug in the term for h and you'll get a function of the price depending on r:

. After few steps of rearranging you'll get:

You'll get an extreme (minimum or maximum) value of the price if p'(r) = 0:

. Now solve p'(r) = 0 for r. I've got: