Adding a series of ten numbers

1. Have a friend choose and write down a single-digit number. (Two digits for experts.)
2. Ask your friend to name and note a third number by adding the first two.
3. Name a fourth by adding the second and third. Continue in this way, announcing each number, through ten numbers.
4. Ask your friend to add up the ten numbers. You will give the answer before he or she can finish:

The sum of all the terms of this series will be the seventh number multiplied by 11.

   Example:

1. If the numbers selected are 7 and 4:
2. The series jotted down is: 4, 7, 11, 18, 29, 47, 76, 123, 199, 322.
3. The seventh number is 76. 11 x 76 = 836
   (use the shortcut for 11: 7 is the first digit, 6 is the third digit;
   the middle digit will be 7 + 6, and carry the 1: 836).
4. So the sum of the ten numbers is 836.