James has a defective clock in his room. The clock is digital, and some of the seven bars on its units digit are broken. A bar that works is on when it should be and off when it should be, whereas a broken bar is off regardless of the time.

Assume that in each scenario, unless otherwise stated, James knows there are broken bars, but doesn't know how many or where they are.

  1. James enters his room and glances at his clock. He knows how many bars are broken but he doesn't know which ones they are. He immediately knows for sure what the time is.
  2. James enters his room and glances at his clock. He knows which bars are broken and immediately knows for sure what the time is.
  3. James enters his room and knows which bars are broken. He stares at his clock for 60 seconds. Now he knows for sure what time it is.
  4. James enters his room. He knows how many bars are broken but he doesn't know which ones they are. He stares at his clock for 60 seconds. Now he knows for sure what time it is.

In each of these four scenarios, what is the maximum number of broken bars, and which might it/they be?


Image shamelessly stolen from Hubble07.

An example of scenario 1 might be 3. If James walked in, and saw a time whose units digit were 8, and bars A, G, and D were burnt out, him knowing that 3 bars were burnt out and not knowing where, he's still be able to deduce that the number is 8. However, this may not be the optimal answer for scenario 1.

There seems to be some confusion about what is meant here, and I hope that clarifies something.

  • 3
    $\begingroup$ The wording doesn't differentiate between what he knows before looking at the clock, and what he learns from looking at it. This makes the puzzle ambiguous. Also, I know of 6 different patterns for digital clocks. There are 12 hour clocks, displaying the hour as a number between 1 and 12, with and without leading zeroes. There are 24 hour clocks, displaying the hour as a number between 0 and 23, with and without leading zeroes, and some of those display midnight as 24:00. $\endgroup$ Sep 5, 2014 at 17:46
  • 1
    $\begingroup$ I'm assuming by "units digit" you mean "of the minutes number"? Additionally, are you looking for answers where he always knows what time it is, or just for at least one specific time? $\endgroup$ Sep 5, 2014 at 18:04
  • 1
    $\begingroup$ @EnvisionAndDevelop In each scenario, given the conditions provided, he knows what time it is by deduction (certain bars lit produce a time that cannot be put into doubt). So how many bars can you break such that you can still tell the time (given any valid time frame of the day)? That's essentially the meaning here. $\endgroup$
    – Neil
    Sep 7, 2014 at 16:49
  • 1
    $\begingroup$ I think there's still confusion. Can you give us a (non-minimal) example of what an answer might look like, and why that's an answer? Specifically, you just told me that we need to tell the time "given any valid time of day", but you told kaine it didn't "have to work with any time", just that "it could be any time". $\endgroup$ Sep 8, 2014 at 12:42
  • 1
    $\begingroup$ Now you're just repeating the confusing thing and asserting it makes sense. How is it possible that it doesn't have to work "with any time", but that it does have to work knowing that it "could be any time"? Those sound the same to me. "If it meets the conditions of the scenario, it is a valid answer." is especially unhelpful, because I'm asking you to clarify the conditions of the scenario. $\endgroup$ Sep 8, 2014 at 13:01

3 Answers 3


You said - "The clock is digital, and some of the seven bars on its units digit are broken." and then at the end you say - "In each of these four scenarios, what is the maximum number of broken bars, and which might it/they be?"

Firstly this should mean that the answers to all the four questions should be an integer in the interval [1,7] since at the most it could be that all the seven bars are broken.

If this is indeed the case then we should only be worried about the single seven segment display which shows the unit place of the two digit minute.

enter image description here

I have tried to do this for the general case i.e. for any time on the digital clock

For Case 1
Edit- As pointed out by @justhalf it looks like for this case there is no solution for which James would be able to figure any time so the strategy given below works for all such times that don't have a 2 or 3 as the last digit.

Since he is not aware which segments are broken the only way he could correctly identify all the digits from 0-9 (except 2 & 3) is if only one segment is broken and that segment is C. For e.g when it is 1-minute only B would be lit and since he knows that one segment is broken then he would conclude that that one segment has to C because if it were any other then it wouldn't form any know digit from 0-9 (except 2 & 3). Similar arguments hold for any minute from 0 to 9 (except 2 & 3). So if only C is broken then James could figure out all time with the exceptions mentioned above.

For Case 2
In this case if he knows that two segments (C & D) are broken then again he can correctly predict all the digits from 0-9.So now the maximum broken segments is 2 i.e C and D.

For Case 3
Here if James knew that there are exactly four broken segments and they are A,C,D and E then he will be able to correctly say the time after 1-minute. There are 10 transitions of state possible in 1-minute which differ by a minute like for e.g 0 minute becomes 1 minute (0→1) or 1→2 and so on. For each of these transitions there is a unique combination of LEDs that will(or will not) change state.

For e,g with A,C,D and E broken lets say 0→1 happens in 1-minute then James would see:-

enter image description here

In the above transition we can see that in 1-minute the LED F changes state (goes from ON state to OFF state) whereas B remains unchanged. So seeing this he can conclude that 0→1 has happened and hence the current time is HH:M1M2 where M2 is found to be 1. Similar argument holds for all other transitions which is summarized in the table below.

Possible Transitions
(Initial minute → final minute)
LED's that change in one minute LED's remaining unchanged in one minute
0→1 F B
1→2 G B
3→4 F BG
4→5 B GF
7→8 GF B
9→0 G BF

So this way James could tell any time displayed on the digital clock. So the maximum broken segments with which he can do this is 4 and they are A,C,D and E.

  • $\begingroup$ For your Case 1, I think if C is broken and the true time is showing the digit '3', James won't be able to tell whether it's a '3' with broken C, or '2' with broken E. $\endgroup$
    – justhalf
    Sep 10, 2014 at 8:13
  • $\begingroup$ @justhalf Yes right I missed that. Now I'm not able to find any general solution to that case and it looks like there isn't any. $\endgroup$
    – Hubble07
    Sep 10, 2014 at 15:28

I will use the A-G bar notation given in the puzzle. I am also assuming that I only need to find one time that works, based on comments.

Case 1: 4 Broken, A, B, D, G.
James walks in and E is lit. There are only 4 digits that use E (0, 2, 6, 8). The digit must be a 2 because the other 3 numbers use at least 6 bars, which would mean that at least 2 bars would have to be lit.

Case 2: 6 Broken, All but C.
Bar C is used for all digits except for 2. James walks in and no bars are lit. He knows the digit must be a 2 and knows what time it is.

Case 3: All 7 Broken.
James walks in and no bars are lit because they are all broken. He waits 1 minute and the minutes tens digit changes. Now he knows it was a 9 and is now a 0, and knows what time it is.

Case 4: 6 Broken, All but C.
Same as case 2. James walks in and nothing is lit. Waits 1 minute and C is lit. Now he knows it was a 2 and is now a 3. Could also use the case 3 reasoning with any one bar not broken.

  • $\begingroup$ According to the clarification in the comments, this is the one that actually answers the right question. $\endgroup$ Sep 9, 2014 at 15:57
  • $\begingroup$ Regarding why the answers are so similar, I suspect the questioner didn't necessarily know what the answers were, and assumed the differences in cases would lead to different (and increasing in number) answers. $\endgroup$ Sep 9, 2014 at 16:00
  • $\begingroup$ Less similar now as I realized an error in case 3. $\endgroup$ Sep 9, 2014 at 18:17
  • $\begingroup$ Shouldn't case 4 also be all 7 broken? (After all, if you know that 7 of the 7 bars are broken, then you also know which of the 7 bars are broken, namely all of them). $\endgroup$
    – celtschk
    Jul 11, 2015 at 11:33
  • $\begingroup$ No, because case 4 stipulates that when walking in he doesn't know which ones. If all 7 were broken, he would know that. $\endgroup$ Jul 11, 2015 at 11:53

I assume that this does not need to work for any time and an AM/PM clock works fine. These all work for 24 hour clocks too. It does not work if there are leading zeros.

If we need to distinguish an 8 from anything else without waiting around, 4 lit lights are necessary. This is because the central light must be lit or it could be a 0. The upper right must be lit or it could be a 6. This leaves 2, 3, 4, and 9. The lower left being lit eliminates 3, 4, and 9 letting upper left or lower right being lit to eliminate 2. This is useful for the right most digit.

2 can be identified from any other number less than 6 with 2 lit lights: the central light and the lower left light. This is useful for the tens digit.

2 lit lights can distinguish 12 from any other options for the first 2 digits. These would be the lower right bar on the 1 and the central bar on the 2.

These altogether means that at exactly 12:28 James can tell what time it is with 20 broken bars. These seems to be the only situation where that works.

1) Knowing how many lights are broken cannot help him any more than this. There still have to be 8 lights illuminated to differentiate between the digits.

2) If he knows which ones are broken, he can use them to identify numbers by unlit bars. This cannot help for the first two digits so the time still starts with 12 and 12 of those 14 bars are broken. For the other two places, knowing that the lower right bar is unbroken but unlit indicates that it is a 2. This means that at 12:22, he can tell what time it is with 24 broken bars.

3/4) For the finale: At the change between 9:59 to 10:00 with a particular arrangement of broken bars, he can tell exactly what time it is with only 1 unbroken bar (27 broken ones) even if he did not know what they are. If the upper right bar of the left most digit changes from being unlit to lit within the 60 seconds, the time changed from 9:59 to 10:00. This is the only time that change can happen for either 12 hour or 24 hour clocks.

  • $\begingroup$ Note that the question has been clarified to need to work for any time so this answer is invalid. $\endgroup$
    – kaine
    Sep 7, 2014 at 17:03
  • $\begingroup$ I didn't said it had to work with any time, I said it could be any time. $\endgroup$
    – Neil
    Sep 7, 2014 at 20:09
  • $\begingroup$ @Neil: So this answer is valid? $\endgroup$
    – justhalf
    Sep 10, 2014 at 8:15
  • $\begingroup$ No, it is correct for the entire time, not just the individual minutes digit. This was the first answer and I think it contributes to the site, but later answers understood more what he wanted. $\endgroup$
    – kaine
    Sep 10, 2014 at 10:27

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