Two hour glasses brain teaser. See above.

Question

Two hour glasses brain teaser.  See above.

Sigiloso · Accepted Answer

Turn both hour glasses over at the same time.  When the 7 minute hour glass is done there will be 4 minutes left on the other hour glass.  Turn this one over (this is that start of the 15 minutes).  Once the 4 minutes has drained, immediately flip it back over and let the 11 minutes drain.  This will give you the 15 minute time you need.

Sigiloso · Answer

A better answer than the one above (because it doesn't require starting the timers before 15 and it can be continuous) would be this:
Flip both timers (Time = 0)
Let the 7 run out with 4 min left on the 11 (Time = 7)
Flip the seven back over (Time = 7)
Let the 11 run out with 3 left on the 7 (Time = 11)
Flip the 7 one more time, giving it four minutes to go. (Time = 11)
Let the 7 run out (Time = 15)
To continue simply repeat

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