A tea seller finds that the number of cups he sells in a day has gone up by 25%, while each cup now holds 12% less tea than before. What is the change in the quantity of tea he sells in a day?

Tea sold = cups $x$ tea per cup, so the two changes multiply: $1.25\times 0.88=1.10$, a rise of 10%. Take 100 cups of 100 ml: 10,000 ml before, and 125 cups of 88 ml = 11,000 ml after. 'A rise of 13%' subtracts the rates (25 - 12) and 'a rise of 37%' adds them; neither works, because the 12% cut bites on every one of the larger number of cups. 'A fall of 12%' quotes the cup change on its own.