Read it through once

We learn also from the narrative, that Tom was an old man, not less than forty-five, and probably fifty years of age. She tells us that Shelby had a son, by name George, who was thirteen years of age; and that Tom was seven years older than his master Shelby. Supposing that Shelby was twenty-five years of age when his son George was born; and that George was thirteen years of age, and that Tom was seven years older than his master, it stands thus: seven added to twenty-five make thirty-two, and thirteen added to thirty-two, make forty-five. But supposing that Shelby was thirty, when George was born, the result would be fifty.