We were looking last week a simple and intuitive demonstration, suitable for functional arithmetics, that the product of two negative numbers is a positive number (less times less is more). And Juan Antonio Adán suggests a not very orthodox, but effective one: “Regarding the first problem, how to explain in a simple way that the product of two negative numbers is positive, it could be like this: if I do not have two friends to whom I owe 15 euros, "I don't have 30 euros less."
