Compound interest

The Monk placed £200 in a London bank in 1968 and then travelled forward two hundred years to pick up a fortune in compound interest. (TV: The Time Meddler)

Symington and Blenkinsop charged compound interest of five minutes per hour per hour on their time loans. Amy Pond, who admitted to having little understanding of economics, misinterpreted this as "five minutes per hour", and so inadvertently racked up a debt of ten years. Fortunately for her, she did not ultimately have to pay this debt, since the Doctor successfully managed to bring down the company. (PROSE: Borrowed Time)