Who am I? number puzzles, step by step

"I am a four-digit number.

The question

What number am I?

Cross out everything the clues rule out

1000to99999000
still standing:9000
90004500
4500100
1007
71
8060
8150
8240
8330
8420
8510
8600
  1. Nothing is given except statements, and the first thing to fix is how much room there is to search: a four-digit number runs from 1000 to 9999, so there are 9000 of them and 4 clues to get down to one.
  2. Do not try to guess a number and check it against the clues — that is 9000 guesses in the worst case. Take the clues one at a time instead, and keep only the numbers still standing after each. The count going down is the working.
  3. First clue: The ones digit is even. That leaves 4500 of the 9000, roughly halving it.
  4. Clue 2: The thousands digit is 8 more than the ones digit. That leaves 100 of the 4500, which is most of them gone.
  5. Clue 3: Its four digits add up to 14. That leaves 7 of the 100, which is most of them gone.
  6. Clue 4: The first two digits add to 13. That leaves 1 of the 7, and that is the one.
  7. Down to a handful the last clue chooses between — 8060, 8150, 8240, 8330, 8420, 8510, 8600 — and it keeps just 8510. Seeing the finalists is the point: the answer is picked from them, not guessed, and the clues could have been taken in any order — only the counting changes.

How it works.

  1. 01

    Nothing is given except statements, and the first thing to fix is how much room there is to search: a four-digit number runs from 1000 to 9999, so there are 9000 of them and 4 clues to get down to one.

  2. 02

    Do not try to guess a number and check it against the clues — that is 9000 guesses in the worst case. Take the clues one at a time instead, and keep only the numbers still standing after each. The count going down is the working.

  3. 03

    First clue: The ones digit is even. That leaves 4500 of the 9000, roughly halving it.

  4. 04

    Clue 2: The thousands digit is 8 more than the ones digit. That leaves 100 of the 4500, which is most of them gone.

  5. 05

    Clue 3: Its four digits add up to 14. That leaves 7 of the 100, which is most of them gone.

  6. 06

    Clue 4: The first two digits add to 13. That leaves 1 of the 7, and that is the one.

  7. 07

    Down to a handful the last clue chooses between — 8060, 8150, 8240, 8330, 8420, 8510, 8600 — and it keeps just 8510. Seeing the finalists is the point: the answer is picked from them, not guessed, and the clues could have been taken in any order — only the counting changes.