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Number Lock Puzzle: Can You Crack the Code?

Number Lock Puzzle: Can You Crack the Code?

The number lock puzzle poses 5 clues on 3 digit codes. From the clues, the correct three digit code that'll open the lock is to be discovered in 10 mins.

Number lock puzzle: Can you crack the code to open the lock?

Following shows five clues based on three-digit codes for a number lock.

Clue 1: Code 291: One digit is right and is in its place.

Clue 2: Code 245: One digit is right and is in the wrong place.

Clue 3: Code 463: Two digits are right but both are in the wrong place.

Clue 4: Code 578: No digit is correct.

Clue 5: Code 569: One digit right but is in the wrong place.

From the five clues, find the three-digit code that will open the number lock.

Time to solve: 5 minutes.

Solution to Number lock puzzle: Can you crack the code?

No decision can be taken using only Clue 1. But Clue 1 and Clue 2 together help to draw the first blood.

Analysis: Digit 2 cannot be in the same leftmost position in 291 and 245 and be the right digit in the right place in 291 and the right digit in the wrong place in 245. This is a contradiction.


Conclusion 1: Digit 2 is wrong and is not in the code.

Now look at the 4th clue on 578 that declares all three digits as wrong. So conclude,

Conclusion 2: As in 245 both digits 2 and 5 are wrong, digit 4 must be the right one and it can occupy only the leftmost or the rightmost position.

Digit 4 again appears in 3rd clue on code 463 that says, two digits are right but both are in the wrong place. Next conclusion made,

Conclusion 3: As leftmost position in 463 and middle position in 245 both are wrong for the right digit 4, correct position of digit 4 must be the rightmost.

The code is in the form: “? ? 4”, with first two digits from left not yet known.

As potential of clue 1 not yet exhausted, revisit it,

Clue 1: Code 291: One digit is right and is in its place.

It is already known that,

  • digit 2 is wrong, and,
  • rightmost position belongs to right digit 4.


Conclusion 4: Digit 1 in 291 is wrong (because of position conflict with digit 4) along with digit 2. The right digit in the code is 9 and it appears in the correct middle position.

This is an important breakthrough.

The code is in the form, “? 9 4” with only the leftmost digit to discover.

Code of 569 of clue 5 includes digit 9. The clue says, the right digit is only one, and in the wrong place. 9 being the right digit and 5 being a wrong one, clue 5 offers us the new information,

Conclusion 5: Digit 6 and digit 5 both are wrong.

Finally, Clue 3 says, two digits of 463 are right and both are in the wrong place. It gives you the critical breakthrough,

Conclusion 6: As digit 6 is wrong and digit 4 is right, digit 3 must be the second right digit and it will take the leftmost vacant place in the code.

The code is 394.

End note

The number lock puzzle is a reasoning puzzle with the clues as conditions. Each clue is processed together with other clues. Selection and processing of the right clue at the right time speeds up the solution.

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