One of the drinks in the bar at the Wayside Tavern is priced at $10 (Ten Dollars).
On a quiet afternoon, one of the regulars is in a school of four, and orders 4 (four) of this drink.
When the barmaid serves up the 4 (four) drinks [@$10 each] the customer asks what the total price will be.
The barmaid smiles sweetly and announces that she'll just have to "...go back to the cash register and check".
.... Back at the cash register she reads the total, looks up and cheerfully announces that "Four of those will cost...er.. $40".
So continues life behind the bar!