Before you start

Pick a book you can borrow from the host’s shelf or a stack of books in the room. Secretly turn to page 108 and remember the 9th word on the page, counting from the top. That is the whole preparation.

The arithmetic

Hand your volunteer paper and a pen, and turn away.

  1. A number. “Write down any three-digit number where the first and last digits are different by at least two.” For example, 742.
  2. Reverse it. “Write the same digits backwards underneath.” 247.
  3. Subtract. “Take the smaller number from the bigger one.” 742 − 247 = 495. If the answer has only two digits, such as 99, it has broken the rule in step 1; have them start over.
  4. Reverse the answer. “Now write that answer backwards.” 594.
  5. Add. “Add those last two numbers together.” 495 + 594 = 1089.

Every number that follows the rule ends at 1089. Do not say so.

The book test

  1. Split the total. “The first three digits are a page number. The last digit is a word on that page.” That is page 108, word 9.
  2. They look it up. Hand them the book, or let them choose one from a stack (if you prepared several books the same way). They turn to the page and silently count to the word.
  3. The reveal. Struggle a little. Name a letter first, then the word. Iris writes it down before they look, so the prediction has been sitting on the table the whole time.

Why it works: after reversing and subtracting, the middle digit is always 9 and the outer digits always add to 9 (for example 495, 198, 693). Reversing and adding such a number always gives 1089.

Keep it from looking like maths

  • Let a calculator do it. On a phone, the arithmetic feels like their own private process rather than a school exercise.
  • Never repeat it with the same person. A second 1089 gives the game away instantly.
  • Do not reveal the number. The word is the reveal; 1089 is just the route there.

Credit where due: The 1089 result is a classic of recreational mathematics. We checked every three-digit number whose first and last digits differ by at least two: all of them end at 1089.

Next: The clock prediction