# Sand Timer Puzzle

This Puzzle is one of the most commonly asked interview puzzle.

Problem:

You have two sand timers with you. One can measure 7 minutes and the other sand timer can measure 11 minutes. This means that it takes 7 minutes for the sand timer to completely empty the sand from one portion to the other.

You have to measure 15 minutes using both the timers. How will you measure it ?

Solution:

Mathematically

7 Minutes Sand Timer Finished.

Time Remaining in 11 minutes timer – 4 minutes

Reversing the 7 minutes timer – 4 minutes will elapse. 3 Minutes will left.

Once 11 minutes gets over reverse the 11 minutes timer again to use that 3 minutes. 8 Minutes left.

Now Reverse 7 minutes timer to measure 7+8 =  15 minutes.

### 0 Thoughts on “Sand Timer Puzzle”

1. Twinkle Prasad on October 5, 2013 at 12:13 am said:

we keep both sand timer together in a position and as 7min timer finished we start measuring time. so 4minutes measured in 11mint timer and after it get finished we will reverse it. so we will get 4+7=15

2. start both the timers. when the smaller one is done, we’ve spent 7 mins. reverse the smaller timer. by the time the bigger timer is done, we’ve measured 11 mins. the smaller timer has spent another 4 mins, meanwhile. reverse the smaller timer again, which has now 4 mins left. when the smaller timer is done fully, 15 mins will have elapsed.

3. Johne14 on July 13, 2014 at 9:57 am said:

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4. Piyush Gupta on August 1, 2014 at 6:49 pm said:

how can reverse the 11 min timer …as 4 mins are left in that timer only …lol

5. jwhite007 on February 7, 2015 at 11:27 am said:

Twinkle Prasad’s solution is correct and better than the one presented by the author. The only problem with Twinkle’s explanation is the equation which he presents. He should have written 4 + 11 = 15… Start both sand timers at the same time. When the 7 minute timer finishes, this is time point 0. There are 4 minutes remaining for the 11-minute timer to drain. When that is done we are only 4 minutes into the 15-minute period, so 11 minutes remaining. Flip the 11-minute timer. After that timer finishes, the 15-minute period will be over.

6. samidh on April 15, 2015 at 7:59 pm said:

Rohit’s solution is much better

7. 11 minutes and 7 minutes sand timer will be started simultaneously.Now first 7 minutes timer will be empty after 7 minutes.
So time elapsed now is 7.
Time left in 11 minutes sand timer is 4.Now again 7 minutes sand timer and remaining 11 minutes timer will be started.So after 4 minutes 11 minutes sand timer will be empty and 3 minutes time of sand remaining in 7 minutes sand timer.
Total time elapsed so far is 11 minutes.
Now 7 minutes sand timer gets reversed.So on reversing 7-3 i.e 4 minutes of sand remaining in the sand.So start emptying the sand of 7 minutes
so total elapsed time will be (11+4)=15

8. Shinigami on August 6, 2016 at 3:02 pm said:

11/0 7/0 Total = 0
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V
4/7 0/7 Total = 0 + 7
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0/11 4/3 Total = 7 + 4
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V
4/7 0/7 Total = 11 + 4 = 15