Christopher and Columbus by Elizabeth Von Arnim

In their berths that night before they went to sleep, it occurred to them that perhaps what was the matter with the stewardess was that she needed a tip. At first, with their recent experiences fresh in their minds, they thought that she was probably passionately pro-Ally, and had already detected all those Junkers in their past and accordingly couldn't endure them. Then they remembered how Aunt Alice had said, "You will have to give your stewardess a little something."

This had greatly perturbed them at the time, for up to then they had been in the easy position of the tipped rather than the tippers, and anyhow they had no idea what one gave stewardesses. Neither, it appeared, had Aunt Alice; for, on being questioned, she said vaguely that as it was an American boat they were going on she supposed it would have to be American money, which was dollars, and she didn't know much about dollars except that you divided them by four and multiplied them by five, or else it was the other way about; and when, feeling still uninformed, they had begged her to tell them why one did that, she said it was the quickest way of finding out what a dollar really was, and would they mind not talking any more for a little while because her head ached.