The HourGlass PUZZLE Two Sand Timers Interview Puzzle
video description
1) Turn both sand timers over and let them run through.
You have measured 4 minutes when the smaller one runs out.
2) Turn the smaller one over again and let the larger one continue to run through.
You have measured 7 minutes when the larger one runs out.
3) Turn the larger one over again and let the smaller one continue to run through.
You have measured 8 minutes because the smaller one has run through twice.
4). and if I'm right that means that just 1 minute of the larger one has run through.
So turn that over again to get 9 minutes
Date: 2023-11-15
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Comments and reviews: 29
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Start both the hour Glass at the same time but don't turn on the timer. When the 4 minutes time is up, one will be empty while other has 3 minutes worth of sand. Immediately flip the 4 minutes hour Glass and let the sand slide. At the end of the next 3 minutes, the bigger glass runs out and now you turn on the timer. Now the 4 minutes glass has a minute worth of sand. After the sand empties, immediately flip it for 2 more times. 1+4+4 = 9 minutes. Not the best method but one of the methods
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Start both the hour Glass at the same time but don't turn on the timer. When the 4 minutes time is up, one will be empty while other has 3 minutes worth of sand. Immediately flip the 4 minutes hour Glass and let the sand slide. At the end of the next 3 minutes, the bigger glass runs out and now you turn on the timer. Now the 4 minutes glass has a minute worth of sand. After the sand empties, immediately flip it for 2 more times. 1+4+4 = 9 minutes. Not the best method but one of the methods
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Revlos
I didn't realise the goal was to get 9 minutes from the start, I thought the point was to setup the timers in such a way as you could time 9 minutes. I did everything the solution said except when I got to having 1 minute left in one hourglass I said to turn them sideways. When you're ready to start just let the 1 minute run, then the 4 minute timer followed by the 4 minute again.
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I didn't realise the goal was to get 9 minutes from the start, I thought the point was to setup the timers in such a way as you could time 9 minutes. I did everything the solution said except when I got to having 1 minute left in one hourglass I said to turn them sideways. When you're ready to start just let the 1 minute run, then the 4 minute timer followed by the 4 minute again.
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Taylor
A) Flip both
B) Flip 4 when it completes - leaves 3 min remain in the 7 --> total 4 minutes has elapsed
C) Flip 7 when it completes, leaves 1 min remaining in the 4 --> 4 + 3 = 7 minutes elapsed now
D) Flip both again when 4 completes - leaves 6 remaining in the 7 --> 4 + 3 + 1 = 8 min elapsed now
E) Lastly, flip 7 back again --> 4 + 3 + 1 + 1 = 9 minutes elapsed.
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A) Flip both
B) Flip 4 when it completes - leaves 3 min remain in the 7 --> total 4 minutes has elapsed
C) Flip 7 when it completes, leaves 1 min remaining in the 4 --> 4 + 3 = 7 minutes elapsed now
D) Flip both again when 4 completes - leaves 6 remaining in the 7 --> 4 + 3 + 1 = 8 min elapsed now
E) Lastly, flip 7 back again --> 4 + 3 + 1 + 1 = 9 minutes elapsed.
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Praveen
Start both the hour glass together at the same time, turn upside down the 2nd one (4 hrs) once 4 hrs elasped, when 7 hrs elasped, turn it (1st one) upside down. The 2nd glass would still have 1 hr left.
When 8 hrs elasped. Stop the 2nd hr glass. The 1st hr glass would still hv 1 hr left. Turn it upside down and wait for 1 hr. 9 hr has exactly elapsed
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Start both the hour glass together at the same time, turn upside down the 2nd one (4 hrs) once 4 hrs elasped, when 7 hrs elasped, turn it (1st one) upside down. The 2nd glass would still have 1 hr left.
When 8 hrs elasped. Stop the 2nd hr glass. The 1st hr glass would still hv 1 hr left. Turn it upside down and wait for 1 hr. 9 hr has exactly elapsed
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Devon
4x4 - 7 =9 start both at the same time and then let the 4 minute hourglass runs four times for 16 minutes and the 7 minute hour glass, runs for seven minutes, you will start to count 9 minutes when the 7 minute hour glass ends. here assuming the 4 minute can reset immediately and will run for 16 minutes (4 times) and the 7 minute will run for one time
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4x4 - 7 =9 start both at the same time and then let the 4 minute hourglass runs four times for 16 minutes and the 7 minute hour glass, runs for seven minutes, you will start to count 9 minutes when the 7 minute hour glass ends. here assuming the 4 minute can reset immediately and will run for 16 minutes (4 times) and the 7 minute will run for one time
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mm
it's so easy. start with the 4 hour glass wait until it ends then again flip it and wait until it ends then flip both the4 and the 7hour glasses and when the 4ends just flip it again and wait until 7hour ends then so you will have a minute on the 4hours glass and totally we will have the 9minutes-
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it's so easy. start with the 4 hour glass wait until it ends then again flip it and wait until it ends then flip both the4 and the 7hour glasses and when the 4ends just flip it again and wait until 7hour ends then so you will have a minute on the 4hours glass and totally we will have the 9minutes-
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chhaya
Wrong answer
The answer is
Start the 7 min hourglass and simultaneously keep 4 min hourglass sleepy
Then after 2 min the second hourglass will be distributed in 2-2 part.
Then after the seven minute has run flip the 4 min hourglass which has 2 min
7+2=9
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Wrong answer
The answer is
Start the 7 min hourglass and simultaneously keep 4 min hourglass sleepy
Then after 2 min the second hourglass will be distributed in 2-2 part.
Then after the seven minute has run flip the 4 min hourglass which has 2 min
7+2=9
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Douglas
This solution assumes that the sand runs through hour glasses run at a constant rate, which I don't think is fair. I think your last step will be longer than a minute, the sand will run more slowly when there is less sand on top of it pushing it through.
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This solution assumes that the sand runs through hour glasses run at a constant rate, which I don't think is fair. I think your last step will be longer than a minute, the sand will run more slowly when there is less sand on top of it pushing it through.
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Henry
Turn both hourglasses over. After 4 minutes, the 7 will have 3 min left. Turn the 4 over. When 7 is empty, the 4 has 1 min left and 7 minutes have elapsed. Turn 7 over, allow it to empty until the 4 is empty (1 min, then turn 7 over for final minute.
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Turn both hourglasses over. After 4 minutes, the 7 will have 3 min left. Turn the 4 over. When 7 is empty, the 4 has 1 min left and 7 minutes have elapsed. Turn 7 over, allow it to empty until the 4 is empty (1 min, then turn 7 over for final minute.
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Naveen. N
Very help for this video-.
Tell me below question answer?
1) I have a 10 bottle each one having 10grm but only one having an 9gram, To check weight machine, minimum number of check the weight for which bottle for 9 grams?
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Very help for this video-.
Tell me below question answer?
1) I have a 10 bottle each one having 10grm but only one having an 9gram, To check weight machine, minimum number of check the weight for which bottle for 9 grams?
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Christian
I came up with: run both same time. At 4 mins run the small one and continue the big one. At 7mins run the big one again while small one continues. At 8mins small finished and flip the big one with one minute to run.
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I came up with: run both same time. At 4 mins run the small one and continue the big one. At 7mins run the big one again while small one continues. At 8mins small finished and flip the big one with one minute to run.
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Kailey
I thought the answer was to start the 4 minute glass first then when it is half way finished so after 2 minutes to start the 7 minute timer and when the 7 minute timer is complete it would have been 9 minutes.
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I thought the answer was to start the 4 minute glass first then when it is half way finished so after 2 minutes to start the 7 minute timer and when the 7 minute timer is complete it would have been 9 minutes.
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syed
It is not optiml.
Optimal would be. frst strt 7 min hourgls. it will take 7 min.
Then strt 4 min hourglss.
As soon as 4 min hour glass gets half emptied. 7+2. u got 9 min
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It is not optiml.
Optimal would be. frst strt 7 min hourgls. it will take 7 min.
Then strt 4 min hourglss.
As soon as 4 min hour glass gets half emptied. 7+2. u got 9 min
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Vijay
Brilliant. By your method we reach the solution in 9mins! Most brains are wired to first get to a point where you have 2mins in the small glass and then start to measure time 2+7.
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Brilliant. By your method we reach the solution in 9mins! Most brains are wired to first get to a point where you have 2mins in the small glass and then start to measure time 2+7.
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if i first put 7 and 4 together down, then when 4 were done, 7 left 3, then reverse both, start calculating, when one was gone, reverse both, continue the move three time?
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if i first put 7 and 4 together down, then when 4 were done, 7 left 3, then reverse both, start calculating, when one was gone, reverse both, continue the move three time?
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Sujit
If we started bigger hourglass after 2minutes of smaller hourglass and then wait for the stop the hourglass. it also measures 9 minutes.
Am i right?
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If we started bigger hourglass after 2minutes of smaller hourglass and then wait for the stop the hourglass. it also measures 9 minutes.
Am i right?
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Jonathan
You can also flip the 4 min timer then when half full turn the 7 min HourGlass upside down when the 7 min HourGlass is finished you get 9 mins EZ PZ
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You can also flip the 4 min timer then when half full turn the 7 min HourGlass upside down when the 7 min HourGlass is finished you get 9 mins EZ PZ
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pratik
Very nice bro excellent -
Till 2: 30 I understood from the website but after that there was confusion which went away after watching your video.
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Very nice bro excellent -
Till 2: 30 I understood from the website but after that there was confusion which went away after watching your video.
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Randy
An hourglass doesn't necessarily run at a constant rate. A seven minute timer, halfway through it's time, isn't NECESSARILY at 3. 5 minutes.
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An hourglass doesn't necessarily run at a constant rate. A seven minute timer, halfway through it's time, isn't NECESSARILY at 3. 5 minutes.
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Rishab
Turn 7 min hourglass until it is finished and then synchronously turn the 4 min hourglass until it is half finished I guess is also correct
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Turn 7 min hourglass until it is finished and then synchronously turn the 4 min hourglass until it is half finished I guess is also correct
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Sachin
Assumption: dont include time taken in turning the glasses because you include that would be around 9 minutes 04 seconds, but good puzzle
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Assumption: dont include time taken in turning the glasses because you include that would be around 9 minutes 04 seconds, but good puzzle
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Amardeep
Why cant u let the 7 mins run on that hour glass, and then start 4 min hour glass, exactly at half way u will know that its 9 mins
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Why cant u let the 7 mins run on that hour glass, and then start 4 min hour glass, exactly at half way u will know that its 9 mins
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mohammed
awesome way to think.
yes i tried it through few other methods but i have to agree that this is the only optimal solution
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awesome way to think.
yes i tried it through few other methods but i have to agree that this is the only optimal solution
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Savitha
you have two hour glasses, one for seven minutes and one for five. How can you use them to measure exactly 13 minutes?
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you have two hour glasses, one for seven minutes and one for five. How can you use them to measure exactly 13 minutes?
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Ankith
i solved it but with a different approach, my solution isn't optimised one, it takes more than 9 mins to measure 9 mins
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i solved it but with a different approach, my solution isn't optimised one, it takes more than 9 mins to measure 9 mins
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SIVAPRADEEP
Just turn the 4 min clock upside down 2 times. and in the third time approximately measure one minitue. -----
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Just turn the 4 min clock upside down 2 times. and in the third time approximately measure one minitue. -----
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pankaj
But at the end for 1 min you have turn both then it becomes 0/7 and 3/1 so finally we counting 9 min as (4+3+1+1)
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But at the end for 1 min you have turn both then it becomes 0/7 and 3/1 so finally we counting 9 min as (4+3+1+1)
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Utsav
Sir, your puzzle are always awesome, mind blowing. and this is much for sharping the brain. Keep doing. - - - - -
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Sir, your puzzle are always awesome, mind blowing. and this is much for sharping the brain. Keep doing. - - - - -
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someone
turn the 4 minute hourglass around then wait till the half drops down and then turn the 7 minute hourglass, done
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turn the 4 minute hourglass around then wait till the half drops down and then turn the 7 minute hourglass, done
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