Puzzle: Sand clock problem
13609 viewsYou have to sand clocks, 1 to measure 4 minutes and the other to measure 7 minutes. How would you measure 9 minutes in exactly 9 minutes?
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By Guest
At 0 minutes, turn both the hourglasses.
Reply 1After 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.
Time 0: Start both timers and run them for 4 minutes.
Reply 2Time 4: The four minute timer has run out, the 7 minute timer has three minutes left in it. Turn over the 4 minute timer, and let the 7 minute timer keep going.
Time 7: The 7 minute timer will run out, and the 4 minute timer will have 1 minute left on it.
Turn the 7 minute timer over and let it run until the 4 minute timer finishes running out. This will be at 8 minutes.
Time 8: Now turn over the 7 minute timer, which will have 1 minute of sand in it, and run it until it runs out