There are two hourglasses. The small one can measure 5 minutes and the large one can measure 7 minutes. How can we measure 16 minutes with 2 hourglasses running together? (If one of the hourglasses finishes leaking, then it must be flipped over immediately to keep it running. )

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Start both hourglasses at the same time. You flip over the small hourglass when it finishes the first 5 minutes. Then you flip over the large one when it finishes its first 7 minutes. When the small hourglass runs to 10 minutes, flip over the small and large hourglass at the same time. It means the large hourglass will run 3 minutes again. When the large hourglass run to 13 minutes (7 + 3 + 3), flip both the small and large hourglass over again to run another 3 minutes.


Anonymous said...

hi, i m vivek from indore sgsits
the both(5&7) hourglasses are leaking .
when the horglass which is 5 minute is over (so that , 2 minutes remaining in 7 minute hourglass ),then start counting
when first time 7 minute hourglass over it's 2 minute counted. then again fill only 7 minute houreglass two times , so that you got 16 minutes [2+7+7 = 16].

Anonymous said...


the ans can be as follows...
start both 7 and 5 together , thn flip 5 as it gets over and when 7 gets over flip 5 at that time 5 will have only 2 min so we will get a total of 7+2=9 min ...use 7 one more time to get the ans....