Hi Surabhi,
Just understand that when you apply this formula to numbers 10-19, your answer is 9, for 20-29, your answer will be 18, for 30-39 your answer will be 27, and so on. Now what has been done is at any given point of time, the figure for all these numbers in 9,18,27,...,99 remains same and same figure is displyed in the crystal ball when you click. So even if you take any 2-digit number, the answer is going to be same figure. When you go for next try, the figures for 9, ....., 99 are changed to any other but same for all. and thus next time you try, you again get the correct answer. Got it??:)