Convert from firkin [butter, soap] to obolos [Ancient Greece]

Unit Definition (firkin [butter, soap])
The firkin [butter, soap] is a traditional unit of weight for butter and soap, equal to 4 stone or 56 pounds (about 25.40 kilograms).

Unit Definition (obolos [Ancient Greece])
The obolos [Ancient Greece] is a historic unit of weight or mass. The obol is a very small weight that originated as the weight of a tiny Greek coin. In ancient Greece the obolos was equal to 1/6 drachma, or roughly half a gram (8 grains).