The solution is simple as you have to think in the right direction.
Start both the sand timers together. Once the 7 minutes are over sand in the 7 minute timer will be completed moved from one portion to the other. Reverse the 7 minutes timer and wait.
When the 11 minutes sand timer gets finished reverse the 7 minutes timer again and wait for it to finish transferring the sand from one portion to another. It will take 4 minutes and in total you can measure 15 minutes.
Mathematically
7 Minutes Sand Timer Finished.
Time Remaining in 11 minutes timer - 4 minutes
Reversing the 7 minutes timer - 4 minutes will elapse.
Once 11 minutes gets over reverse the 7 minutes timer again to use that 4 minutes
So total = 7+4+4 = 15 minutes.
3 comments:
I believe there is a better solution for this problem.
1. Run the 7 min and 11 min clock at same time
2. Once the 7 min clock is done, the 11 min clock have 4 mins to go. Consider this time as zero.
3. Wait till 11 min clock is done and flip it. Once it goes empty, you'll have your 15 mins.
Mathematically: (11-7) + 11
I feel karma243's solution is better one.
Yes Karma solution is good and it reduces some complexity.
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