2
$\begingroup$

You have two hour glasses: A seven minute one and an 11 minute one. Using just these two hour glasses, accurately time 15 minutes.

Improve this question
$\endgroup$
2
  • $\begingroup$ but I do remember a question which asked a similar thing... not sure where and searches dont give me anything $\endgroup$
    – skv
    Nov 26, 2014 at 11:03
  • 1
    $\begingroup$ It's not too far off being a water-jug challenge, no? $\endgroup$
    – Hazel
    Nov 26, 2014 at 11:06

2 Answers 2

Reset to default
3
$\begingroup$

You can try

1. Start both hourglasses.
2. When the seven minutes runs out turn it.
3. When the eleven minutes runs out, turn the seven minute hour glass again. It now has 4 minutes of sand.
4. When the seven minutes hourglass runs out, you will have 15 minutes.

The method is dependant on the accuracy of the hour glasses and your speed at turning them. Not 100% accurate.

Improve this answer
$\endgroup$
1
  • 1
    $\begingroup$ +1 because this is more clearly expressed than skv's answer, albeit later :-) $\endgroup$
    – Rand al'Thor
    Nov 26, 2014 at 12:22
2
$\begingroup$

Start the 11 minute hourglass and the 7 minute hourglass at the same time, as soon as the 7 minute one runs out start timing, once it fully runs down run it again for the full time you would be 15 minutes by the end of the timing

Improve this answer
$\endgroup$
2
  • 3
    $\begingroup$ You can also turn the 11 minute hourglass sideways to stop sand flowing at the point where the 7 minute hourglass is empty, if you need to be able to time 15 minutes starting from some arbitrary point. $\endgroup$
    – Hazel
    Nov 26, 2014 at 11:05
  • $\begingroup$ wow its also a nice way!!!!!!1 $\endgroup$
    – Enigmo
    Nov 26, 2014 at 11:10

Not the answer you're looking for? Browse other questions tagged or ask your own question.