How do the boys figure out that the money lies within the Navaro Graveyard at the center of the city?

Gardo remembers that the prisoner number on the envelope had not matched Gabriel Olondriz's prisoner number, so the boys surmise that maybe that number means something else. They then use that knowledge to figure out that the map and the number are connected. Looking at the map, they use the numbers as coordinates, ultimately determining that the money must be buried somewhere in Navaro Graveyard at the center of the city.