This is a mathematical trick to determine the year of birth, or whatever number you want to aim for.

"I want you to pretend that you're by your telephone and that you're calling me up to give me a hint as to your age. Please look down at your invisible phone and give me four of the digits in your telephone number. Mix them up any way you want and create another four digit number."

Wait till people are done.

"Now subtract the smaller number from the larger."

"Now add up the digits in your answer."

"If your answer has more than one place, add the remaining digits together. Keep doing this until you have one digit."

"Add 7 to this."

"Finally, add to this the last two digits of the year your were born.

"Tell me the total."

(You subtract 16 from the total, and the answer is the year born.)

WHY IT WORKS

When you reduce to a single digit, you will always get 9. When you add 7, you will get 16. So just subtract 16 from the total answer, and you have the year of birth.