Groundwater Hydrology

In a Wenner electrical resistivity sounding survey for groundwater exploration, four equally spaced electrodes have an electrode spacing of $a = 10.0\text{ m}$. If the measured electrical resistance is $R = 2.5\text{ }\Omega$, calculate the apparent resistivity of the subsurface stratum in $\Omega\cdot\text{m}$ using $\rho_a = 2 \pi a R$. (Take $\pi = 3.1416$)