I Need Help ASAP! I have to fig out a way to mesure 15min of time, using two hour glasses......?

heres the acutal question!!!



Problem: You have figured out how to make a compound that can cure cancer (Congratulations!). However, to do so, you need to perform a chemical reaction for exactly 15 minutes. Unfortunately, your laboratory is ancient (thanks to budget cuts at the National Institutes of Health) and all you have at hand is two hour glasses. For one of the hour glasses, the sand will run through in 11 minutes; the sand in the other hour glass will run through in 7 minutes. Explain how you can use these two hour glasses to measure exactly 15 minutes and cure cancer forever. Note: these hour glasses are sealed, therefore you may not transfer sand between them

I Need Help ASAP! I have to fig out a way to mesure 15min of time, using two hour glasses......?
First, let the hour glasses run at the same time.

Of course, the sand in the '7 minutes' hour glass will run through first.

As for the '11 minutes' hour glass, the sand will run through in another 4 minutes.



(Time passed: 7 minutes)



So, the next step is to turn the '7 minutes' hour glass upside down for it to start running again.

Wait until the '11 minutes' hour glass to finish running, which will take 4 minutes.



(Time passed: 11 minutes)



Finally, turn the '7 minutes' hour glass upside down again. It will take 4 minutes to run through.



(Time passed: 15 minutes)



There u have it, try to draw out a diagram so that u can understand better.
Reply:Tsubasa is wrong on this! When you turn the 7min glass over the second time it has 3 MINUTES LEFT NOT 4!!! 7-4=3! Report It
Reply:Correct Answer:

Flip both over at same time. When the 7 min glass finishes start the test/boiling the egg. Let the 11min glass finish draining (4 minutes gone by)and flip over immediately and let drain (15 minutes gone by) Report It
Reply:start them both at the same time....when the 7 minute hour glass empties there will be exactly 4 minutes left on the 11 minute hour glass and that's when you start timing...after that 4 minutes has finished turn it over again and wait for it to empty again (which would be 11 minutes added to the 4 minutes that has already passed) and that will give you exactly 15 minutes :-)
Reply:Start both hour glasses, when the 7 min is empty; flip it, then when the 11 min empties flip the 7 min again when the seven empties again 15 min is up. This method is exactly 15 min from start to finish, there is no waiting to start timing you start when the glasses are turned over