Test your skills of deduction with the August 2021 edition of Riddle Me This: Two Hourglasses
There is a table in front of you with two hourglasses. One contains seven minutes of sand, and the other contains eleven minutes of sand. Using just these two hourglasses, how can you accurately time fifteen minutes?
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start both hourglasses simultaneously. when the 7 minute hour glass has run out, turn over the 11 minute hour glass, which will now run for 4 minutes, then turn it over again for another 11 minutes or 15 minutes total
So according to your solution, 7-4=4? Must be some new math!
Something is incorrect……..”Now flip the seven-minute hourglass and start it again at the same time you turn the eleven-minute hourglass back up, running the four minutes out of it. Once the eleven-minute hourglass is empty, turn the seven-minute hourglass sideways. You now have four-minutes of sand in the seven-minute hourglass…..” If I run a 7 minute hour glass for the four minutes left in the 11 minute hour glass, that would mean I would only have 3 minutes left in the 7 minute hour glass. 7-4=3.
Simple solution would be to turn both over at the same time. When the 7 minute hour glass runs out, then start the clock at 0. when the four minutes run out of the 11 minute hour glass, turn the 11 minute hour glass over. When that runs out, that is your 15 minutes. 4+11=15.
zYour answer is overly complex, Start both glasses, when the seven runs out stop the 11 minute which will have 4 minutes left. To time the fifteen minutes just finish the 11 minute, and then flip it again and do the eleven minutes again.
7-4=3. You only have 3 minutes of sand left in the 7-minute hourglass. The answer I found is to run the 11-minute glass and 7-minute glass until the 7 minute runs out. Then you have 4 minutes in the 11-minute glass. Run those 4 minutes out, then flip the 11-minute glass back over. 11+4=15.
Flip both hourglasses. When the seven-minute hourglass runs out, turn the eleven-minute hourglass on its side. There are four minutes remaining in the eleven-minute hourglass. Now flip the seven-minute hourglass and start it again at the same time you turn the eleven-minute hourglass back up, running the four minutes out of it. Once the eleven-minute hourglass is empty, turn the seven-minute hourglass sideways. You now have four-minutes of sand in the seven-minute hourglass.
NO, you only have three minutes of sand left in the seven-minute hourglass!
Now all you need to do is start the eleven-minute hourglass and as soon as it is empty you flip the seven-minute hourglass back up and run the four additional minutes out, bringing you to fifteen minutes.
… bringing you to fourteen minutes.
A better answer:
Flip both hourglasses. When the seven-minute hourglass runs out, turn the eleven-minute hourglass on its side. There are four minutes remaining in the eleven-minute hourglass.
Now, when you want to time the fifteen minute period, restart the eleven-minute hour glass and run the remaining four minutes out and immediately flip it over and run eleven minutes out, for fifteen minutes total.
Isn’t there a much simpler way to measure 15 minutes? Start both timers at the same time. When the 7-minute timer runs out flip it over. When the 11-minute timer runs out, four minutes have passed and there is four minutes of sand at the bottom of the 7-minute timer. So just flip the 7-minute timer over and when the sand runs out you are at 15 minutes.
That’s the right answer. I agree. You don’t lay an hour glass on it side and the complete process is exactly 15 minutes.
Your solution is more complicated than needed. Simply start both hour glasses. As soon as the 7-minute glass runs out, immediately turn it back over while allowing the remaining 4 minutes to run out of the 11-minute glass. As soon as the 11-minute glass runs out, turn the 7-minute glass back over. When the 7-minute glass empties 15 minutes have transpired. Not only is your solution more complicated, it is less accurate since you are stopping the hour glasses; thereby not truly capturing 15 minutes. In fact, your solution captures a time of 26 minutes, not 15.
…your “corrected” answer is still incorrect…
Your updated answer is still wrong. If you flip the 11 after the 7 minutes has finished it will run for 7 minutes, not 4. Jay B Stevens has the correct answer. Flip both. The fifteen minutes is from the time the 7-minute timer runs out + an additional flip of the 11 once it has finished. (11-7)+11=15
David Wasserman’s answer is correct. While the posted answer works, you have added 7 minutes before you can start timing the 15 minutes. This it adds up to 22 minutes total. David’s is 15 minutes start to finish.