how can a chef use an 11-minute hourglass and a 7-minute hourglass to time vegetables that must steam for 15 minutes (consecutively)?

Question

how can a chef use an 11-minute hourglass and a 7-minute hourglass to time vegetables that must steam for 15 minutes (consecutively)?

Answer 1

turn the 7-minute hourglass upside down so that it starts to empty and turn the 11-minute hourglass upside down so that it starts to empty, but don't turn on the vegetables just yet

 

when the 7-minute hourglass is empty, there are 4 minutes left in the 11-minute hourglass

 

turn on the vegetables and let the 11-minute hourglass run out the last 4 minutes

 

turn the 11-minute hourglass upside down again and let it run out for 11 minutes more

 

the vegetables therefore get steamed for 4+11=15 minutes

 

Answer 2

This is a little tricky.

Turn over both hourglasses at the same time, but don't put in any vegetables. When the 7 minute timer runs out, put in the vegetables. There will be 4 minutes left on the 11 minute timer. When that runs out, just flip it back over for another 11 minutes. That gives you the 4 minutes plus the 11 minutes which is 15 minutes.