Water Money

Denise should receive £7, and Malcolm £1.  Suppose the cups they used each have a capacity of a third of a litre.  Then because there were eight litres in total, each of the three drank eight cupfuls of water that day.  Denise had a total of fifteen cupfuls, having eight to herself and giving seven to Joel.  Malcolm, on the other hand, gave only one cup to Joel out of the nine that he could make out of his supply.