Continuous multiply single digit numbers.
Magician asks for audience's calculator. He key-in 6*3*any number from the audience* then hand-over the calculator to the audience. The audience is asked to continue multiplying number until the total answer is a 7-digit number.
Audience is asked to cover a digit number, except zero, from the total answer and that is his secret number. He tell all the remaining numbers to the magician. The magician "guest" the secret number.
Secret: The sum of the digits of "The product of number multiply by 9" is always divisible by nine.
