Who am I? number puzzles, step by step
"I am a four-digit number.
What number am I?
Cross out everything the clues rule out
- 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.
- 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.
- First clue: The ones digit is even. That leaves 4500 of the 9000, roughly halving it.
- Clue 2: The thousands digit is 8 more than the ones digit. That leaves 100 of the 4500, which is most of them gone.
- Clue 3: Its four digits add up to 14. That leaves 7 of the 100, which is most of them gone.
- Clue 4: The first two digits add to 13. That leaves 1 of the 7, and that is the one.
- 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.
- 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.
- 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.
- 03
First clue: The ones digit is even. That leaves 4500 of the 9000, roughly halving it.
- 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.
- 05
Clue 3: Its four digits add up to 14. That leaves 7 of the 100, which is most of them gone.
- 06
Clue 4: The first two digits add to 13. That leaves 1 of the 7, and that is the one.
- 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.