If you have two sand glasses One of them is for the 4 minute another one - is for 7 minutes If you have to measure 9 minutes with these two sand glasses in 9 minutes?
At 0 minutes, turn both the hourglasses.
After 4 minutes:
The 4 minute hourglass: 0 minutes left.
The 7 minute hourglass: 3 minutes left.
Turn the 4 minute hourglass.
After 7 minutes:
The 4 minute hourglass: 1 minute left.
The 7 minute hourglass: 0 minutes left.
Turn the 7 minute hourglass.
After 8 minutes:
The 4 minute hourglass: 0 minutes left.
The 7 minute hourglass: 1 minute left.
Turn the 7 minute hourglass.
After 9 minutes:
The 4 minute hourglass: 0 minute left.
The 7 minute hourglass: 0 minutes left.
Now you know you are finished.
2 comments:
Your solution does not appear to me to work. You turn the 7 min timer twice. Two times seven is 14.
Here is a simpler solution:
Turn the 4 min and delay the seven minute until the 4 min reaches the half way point then turn the 7 min. Seven and two equals nine.
Easy peasey!
in sand glass you cannot identify the mid point. So you don't have liberty to turn the clock after two mins. First solution is correct
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