Think of two numbers e.g. 4 and 5. We will do three steps.
In a variation which I did with second graders in New York, they picked two numbers from a stack of cards. They showed the first number but not the second one. Before that they had done the three steps with the two numbers.
Use the 4 and 5 to make 45. Use the 4 and 5 to make 9 (4 + 5). Finally find the difference between 45 and 9.
After demonstrating that if they tell me the first number then I will be able to read their mind for the second and do all the three steps and giving the right answer each time, we began to record the findings.
Many of the participants saw the multiple of nine trick. As I pressed for more observations, another interesting one is how the second digit in the answer is derived by subtracting the first number from ten.
I was reminded again that asking students to explore more ways to often result interesting and outstanding responses.
I did this problem with the teachers group as well as the principals group and on both days we got equally interesting responses except I was surprised that no one in the teachers asked for the reason why the trick worked. One did in the pricipals group.