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Solving a 4-digit Hard code, step by step

Seven clues, four digits, zero luck. This is the size of every Daily Challenge, so it's worth getting comfortable with.

The puzzle

Try it yourself first if you like. The answer is at the bottom.

Step by step

1. Clear the board. 5092 says nothing is correct: 5, 0, 9 and 2 are out.
   5092NothingNothing is correct
2. Resolve the fork. 9805 has one digit in its exact spot. 9, 0 and 5 are out, so it's 8, in the second spot.
   9805One correct & well placedOne number is correct and well placed
3. Two for one. 9375 has two correct digits. With 9 and 5 out, they must be 3 and 7. Both are wrongly placed: 3 is not second, 7 is not third.
   9375Two wrong spotTwo numbers are correct but wrongly placed
4. Squeeze out a digit. 7369 also has two correct digits. 3 and 7 are already in and 9 is out, so 6 is out. And from their spots here: 7 is not first, 3 is not second.
   7369Two wrong spotTwo numbers are correct but wrongly placed
5. Find the last digit. 1206 has one correct digit. 2, 0 and 6 are out, so it's 1, and 1 is not first.
   1206One wrong spotOne number is correct but wrongly placed

You now know the four digits are 3, 8, 1 and 7, with 8 in the second spot. Time to place the rest:

The last two clues, 4359 and 1062, were never needed. Run 3817 past them anyway: in 4359 only 3 is in the code, and it's not in the second spot ✔. In 1062 only 1 is in the code, and it's not in the first spot ✔.

What to take away

• Order matters. The same seven clues are a slog read top to bottom, and quick when you start with “nothing is correct”.
• Some clues are spare. Bigger puzzles carry extra clues. Don't feel you have to use them all; use them to double-check.
• Track “not here” per digit. The final placement is always a little table like the one above.

Put it into practice

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