You have a large supply of very rusty $2 \mathrm{in}$. sch 40 steel pipe, which you want to use for a pipeline. Because rusty metal is rougher than clean metal, you want to know its effective roughness before laying the pipeline. To do this, you pump water at a rate of $100 \mathrm{gpm}$ through a $100 \mathrm{ft}$ long section of the pipe and find that the pressure drops by $15 \mathrm{psi}$ over this length. What is the effective pipe roughness in inches?