Take the formula "five" as an example. Whether it is five or five, the formula is, 15 is five.
Two fives add up to ten, which is simply 25 10.
By analogy, three fives add up to fifteen, three five fifteen. Four fives add up to 20, and the formula is 4520. Five fives add up to twenty-five: five five twenty-five.
In this way, the result of adding the same numbers is catchy, concise and easy to remember in the form of formulas.