The Lost Mine — Agatha Christie

Hercule Poirot always kept a healthy symmetrical balance in the bank of four hundred forty-four pounds, four and four pence. But what about investments? Hastings proposes he buy shares worth three hundred pounds in oil fields promising a yield of 100% in one year. No need for speculative behavior, as Poirot explains, for the exercise of his little grey cells was rewarded with fourteen thousand shares by the Burma Mines, Ltd. Hastings wants to know more...