Recently, our Grade 5/6 students tried to solve a puzzle where the letters A-Z = the numbers 1-26.
Most students figured out that you must first assign a number to each letter. Then, the sum of the 2 numbers on the right, minus the sum of the 2 numbers on the left, was equal to the number in the middle.
But, one student discovered that you can also subtract diagonally! This required the class to think about and work with negative integers, but it worked, for all 4 puzzles!