23 replies to this topic

[ChuCkY]

Here's the solution...

1. First, run both the sand-glasses..

2. After 7-min, flip the 7-min sand glass and let the 11-min flow..

3. Again after 4 more minutes, flip the 7-min sand-glass again..

Thats it !! 7+4+4=15..

[BillyAsh]

Sand-glass II. - Back to the Water and Weighing Puzzles
A teacher of mathematics used an unconventional method to measure time for a test lasting 15 minutes. He used just a sand-glass, which spills in 7 minutes and a second sand-glass, which spills in 11 minutes. During the whole time he turned sand-glasses only 3 times. Explain how the teacher measured 15 minutes.

Spoiler for Solution

Sand-Glass II. - solution
When the test began, the teacher turned both 7min and 11min sand-glasses. After the 7min one spilt its last grain, he turned it upside down (the 11min one is still to spill sand for another 4 minutes). When the 11min sand-glass was spilt, he turned the 7min one upside down for the last time. And that’s it.

All very good I see that it works - just one problem you turn the sand glass over four times not three - twice to get the 7 & 11 going then when the 7 finishes you turn that once more and that is now three - when the 11 runs out you need to turn the 7 back over to get the four minutes that it has just used and that makes four turns.

Samuelpt you seem to be ignoring the two times to set both A11 and B7 off

[Snig]

Wouldn't it be better to simply turn both sand timers, 7 minute and 11 minute, and start the test when the 7 minute runs out, leaving 4 minutes on the larger timer. When the 11 minute timer empties, turn it again adding another 11 minutes to the four. Only two turns.

[NintendoMad]

Is it...

Spoiler for Maybe...

1. Turn them both over so they start to spill
2. when the 7 min timer is up, turn that one back over. there will be 4 mins left in the 11 min one
3. when the 11 min timer is finished, in the 7 timer it will be 3 mins at the top and 4 at the bottom. You need 4 more mins, so you turn it back over so the 4 mins (in the 7) will spill back.
so it will be the 11 mins (from the 11 timer) and the remainder 4 mins (from the 7 timer)