P.O. Box - La Plata Mystery Cache

This cache series is being extended and was originally developed by Maryland cacher mdsurveyor. With his kind permission and help, I am continuing the concept.

The posted coordinates are for the Post Office. The cache is NOT in or next to the building. The cache will be within walking distance or only a short drive from the Post Office.

A simple puzzle once you learn how to apply the sets of three numbers for the cache and some simple math. (Grid:Row:Column)

P.O. Box - La Plata
To find the cache location, use the (Grid:Row:Column) format.
N 38° 31.((G1011:R10:C3)+(G21:R1:C3)) W 076° 58.((G1011:R1:C27)-(G21:R3:C1))

The P.O. Box lobby is open 24 hours. The cache is a short walk from the
Post Office.

If this is your first "P.O. Box" cache, visit the first in the series, P.O. Box – Bel Air (GC1BF0T), for the instructions to help you
solve the puzzle.