Solution to the 30 Minute Puzzle – Sand watches

Here is the solution for the 30 minutes puzzle using sand watches posted on 10/24/07. Those who havent yet read the problem, first give it a try at 30 Minute Puzzle – Sand watches.

Run the 13 minute hourglass.
Then start the 13 minute hourglass and the 9 minute hourglass. After 22 minutes, the 9 minute hourglass will be empty. Start(Invert) it again immediately.
At 26 minutes, the 13 minute hourglass will be empty, and the 9 minute hourglass will have… 5 minutes of sand remaining, and 4 minutes of sand expended.
Immediately turn(invert) the 9 minute hourglass.
This means that at 26 minutes, you now have the 9 minute hourglass with 4 minutes of sand remaining.
This will empty exactly at the 30 minute mark.