Ground rod resistance: PE Power General Applications practice question

A 480 V service for a rural pump station is grounded with one 5/8 in diameter, 10 ft copper-clad steel rod driven vertically to its full length. A four-pin Wenner test at the site gives a uniform soil resistivity of 15,000 Ω·cm. The engineer wants to know whether this single rod alone reaches the 25 Ω value named in the NEC.

Using Dwight's formula, the resistance to remote earth of the single rod is most nearly: