Mathematical Idea - Corsican Sums
A corsican sum is a sum where, whenever one ammount is altered then another sum or sums is automatically altered. In other words, one sum magically effects the other.
You have 4 and 5 infront of you. Whenever you subtract from one, 2 is magically added to the other. So 4 - 1 means 5+2 or 4 - 2 means 5 + 4.
You have 1 and 2 and 3 in front of you. Whenever you add 1 to the first then 2 is added to the second and 3 to the third.
Why call them Corsican Sums? : Alexander Dumas wrote a Novel called The Corsican Twins about two twins who could feel each others pain. Like that, the numbers in these sums are automatically altered whenever their "Twins" are altered.