How do you know when 30 minutes is up?

Question

You have exactly 30 minutes to get out of a room. All you have is a 9 minute hour glass and a 13  minute hour glass to count off exactly 30 minutes.  

Answer 1

The way to do this is to start both timers.  Turn over the 9 minute timer as soon as the sand runs down.  Then after the 13 minute timer sand runs down, turn over the 9 minute timer.  At this point, the 9 minute timer will have 4 minutes of sand on top.  After the 4 minutes of sand runs down, start the 13 minute timer to get the remainder of the 30 minutes.

Answer 2

Here is Will N's solution which matches Steve C's solution.  Bethany has posted this question 3 times:

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The key is to note that 13-9=4 and 13+4+13=30.
 
At the beginning of the thirty minutes, start both hourglasses.
 
After nine minutes, the 9 minute hour glass will have emptied. Flip it over immediately.
 
In four more minutes, 13 minutes total will have elapsed. You will know because the 13 minute hour glass will empty. At this point, the 9 minute hourglass has four minutes worth of sand in its lower chamber. Flip it immediately.
 
It will take four minutes to empty. At that point, 17 minutes total have elapsed and there are 13 minutes left.
 
Start the 13 minute hourglass immediately. When it empties, the time is up.
 
I hope this helps.

Answer 3

Bethany,

 

Let the 13 minute hour glass measure the first 13 minutes leaving the 9 minute one alone.

Next set them both going.  After 9 minutes are gone turn the 9 minute hour glass over and let the the remaining 4 minutes of the 13 minute glass run out.  Time elapsed is 26 minutes.  The 9 minute timer has four minutes elapsed...turn it over and when it has run out 30 minutes are up. 

 

 

 

Answer 4

If we combine the hour glasses, we add the minutes.
 
9 minutes + 13 minutes = 22 minutes
 
 
You have 30 minutes and the hour glasses have 22 minutes.
 
30 minutes - 22 minutes = 8 minutes

 
You will know when 30 minutes is up after 8 minutes have passed the moment all the sand in the hour glass runs out.

Answer 5

I would start the 13 mins, then start it over immediately again, thus I would realize that 26mins and a couple of seconds have passed and I have less than 4mins left. I would then start the 9mins until it is about a third of the way. This will be about the three minutes mark. 26+3=29mins. Then I would have a minute allowance that would accommodate the seconds it took to turn over/change hour glasses and getting out of the room, which should give me 30mins. My two cents.